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Our planet is capable
of unleashing extreme chaos.
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00:00:08,945 --> 00:00:16,211
Volcanoes, earthquakes,
hurricanes, and floods
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00:00:16,245 --> 00:00:18,911
can cause untold devastation.
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00:00:20,111 --> 00:00:23,045
We may think we've seen
the worst
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00:00:23,078 --> 00:00:24,411
Mother Nature can throw at us,
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00:00:24,445 --> 00:00:28,478
but scientists struggling
to understand these disasters
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00:00:28,511 --> 00:00:31,311
are discovering evidence
that even more extreme events
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have struck in the past.
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So this is about 13 times
more powerful
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than the Pompeii eruption.
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They're uncovering clues
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00:00:42,678 --> 00:00:45,011
that the worst catastrophes
in history
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00:00:45,045 --> 00:00:47,178
could strike again.
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00:00:50,078 --> 00:00:56,478
Thousands of years ago,
floods of unimaginable violence.
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00:00:56,511 --> 00:00:58,145
This water came up 800 feet.
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That's huge.
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Floods powerful enough
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00:01:01,945 --> 00:01:03,911
to blast through miles
of solid rock
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in just hours.
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But how?
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Everything in this landscape
was screaming
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00:01:11,011 --> 00:01:13,911
in terms of its signs or clues
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that this was made
by catastrophic flooding.
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The clues to some of the biggest
floods ever are here,
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carved in mysterious
rock formations,
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buried beneath the waves,
or hidden in plain sight,
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all around the world.
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Now, scientists find new clues
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to understand
our volatile Earth...
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And unravel the secrets
of "Killer Floods,"
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right now, on "NOVA."
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Major funding for "NOVA"
is provided by the following:
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Floods.
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Events of such violence,
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they turn oceans, rivers,
and lakes
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into devastating walls of water.
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00:02:28,218 --> 00:02:32,551
On average, around the world,
these powerful surges
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kill 25,000 people every year.
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00:02:36,251 --> 00:02:41,151
In 2004, a deadly tsunami
hits Southeast Asia,
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leaving over 200,000 people
dead,
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and $10 billion worth of damage
in its wake.
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00:02:48,351 --> 00:02:52,018
More recently, in 2017,
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Hurricane Harvey
slams into Houston.
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Heavy rains cause catastrophic
flooding,
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killing more than 70,
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and leaving tens of thousands
homeless.
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00:03:02,351 --> 00:03:04,084
That same year,
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a third of Bangladesh
is submerged by flooding
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which extended
throughout South Asia,
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including Nepal, India,
and Pakistan.
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The flooding, caused by
an especially strong monsoon,
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is thought to be the most severe
in the last hundred years.
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But could Mother Nature
have unleashed floods
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that were even bigger
and more destructive
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00:03:27,984 --> 00:03:29,518
in the past?
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That's what a series
of discoveries is suggesting.
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Scientists are unearthing
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00:03:36,351 --> 00:03:40,218
what looks like
the scars of cataclysmic floods
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that dug deep into the rock,
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reshaping the surface
of the Earth itself.
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It completely changed the
face of the landscape.
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No one has ever witnessed
anything
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00:03:55,018 --> 00:03:56,351
even close in scale.
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00:03:56,384 --> 00:03:59,218
Across the world,
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00:03:59,251 --> 00:04:03,151
three far-flung locations
share an eerie similarity.
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In the United States,
16,000 square miles
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00:04:07,751 --> 00:04:10,118
of dry canyons
and bizarre rock formations
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00:04:10,151 --> 00:04:11,584
cover the Northwest.
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00:04:13,084 --> 00:04:15,684
In Iceland,
a 300-foot-deep gorge
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00:04:15,718 --> 00:04:19,384
appears to have been ripped out
in an instant.
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00:04:19,418 --> 00:04:24,118
And off the coast of Britain,
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00:04:24,151 --> 00:04:27,351
a network of mysterious canyons
carved deep into the sea bed
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could reveal how this channel
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00:04:29,284 --> 00:04:33,451
first separated
what is now Britain from France.
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Far from eroding gradually,
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00:04:37,551 --> 00:04:39,751
there's evidence
that vast deluges
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00:04:39,784 --> 00:04:44,018
tore out these landscapes in
the geological blink of an eye.
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00:04:44,051 --> 00:04:49,618
But what could have triggered
such killer floods?
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And could one strike again?
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00:04:57,484 --> 00:04:59,918
The trail of clues starts here,
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on the plains
of Washington state.
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A flat expanse, stretching
for hundreds of miles,
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until suddenly, the
landscape changes.
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Flat fields give way
to sheer gorges,
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some almost a thousand feet
deep.
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Rock islands rise to the height
of 30-story buildings,
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00:05:28,551 --> 00:05:30,451
while in other places,
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strange round depressions,
like gargantuan potholes,
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plunge 50 feet.
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These are the Scablands,
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named by settlers who thought
the formations
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resembled scabs or wounds
on the rocky terrain.
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00:05:51,084 --> 00:05:54,218
Located over a hundred miles
east of Seattle,
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00:05:54,251 --> 00:05:57,151
this mysterious landscape
covers an area
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around 16,000 square miles.
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00:06:04,684 --> 00:06:06,084
For over a century,
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00:06:06,118 --> 00:06:10,284
geologists have been trying
to understand
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00:06:10,318 --> 00:06:13,284
what forces created
the Scablands.
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When you encounter a landscape,
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it's not unlike a detective
encountering a crime scene.
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In the case of this landscape,
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there are features
that act like clues.
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But spotting those clues
takes a trained eye.
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00:06:40,984 --> 00:06:42,984
You can't really get
a sense of this area
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unless you get up high.
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This part of the Scablands
is, like, 30, 40 miles across.
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It's on a mega scale.
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It's the kind of scale that
first led geologists
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00:07:01,818 --> 00:07:05,884
to suspect that the Scablands
had formed slowly,
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00:07:05,918 --> 00:07:11,051
eroded over millions of years
by rivers, wind, or ice.
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00:07:13,218 --> 00:07:16,684
During past ice ages,
as temperatures plummeted,
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00:07:16,718 --> 00:07:18,918
giant ice sheets and glaciers
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00:07:18,951 --> 00:07:21,051
carved deep valleys
through solid rock.
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00:07:22,984 --> 00:07:26,784
Like these, in Glacier
National Park, in Montana.
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And rivers,
scouring rock over eons,
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helped carve some of the most
dramatic landscapes on Earth,
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like the Grand Canyon
in Colorado.
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00:07:41,851 --> 00:07:45,784
But mapping sediments
left behind by the ice sheet
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00:07:45,818 --> 00:07:49,218
when it melted 12,000 years ago,
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shows the ice only made it
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00:07:51,784 --> 00:07:55,384
to the northern edge
of the Scablands.
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And Vic Baker's bird's eye view
reveals
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00:08:01,984 --> 00:08:04,651
that fast-flowing water
was the culprit.
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00:08:06,784 --> 00:08:08,618
The clue?
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00:08:08,651 --> 00:08:12,418
This curved canyon.
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00:08:12,451 --> 00:08:14,051
Looking at this from the air,
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00:08:14,084 --> 00:08:17,151
you can see that the shape
is like a horseshoe,
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00:08:17,184 --> 00:08:22,051
which is what forms
in waterfalls.
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Niagara Falls
and many big waterfalls
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00:08:26,551 --> 00:08:28,751
have a similar horseshoe shape.
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00:08:31,018 --> 00:08:36,051
For that reason, this canyon
is called the Dry Falls.
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But stretching
three and a half miles,
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this formation is five times
the span of Niagara
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and twice as tall.
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00:08:49,284 --> 00:08:53,451
The cliffs behind me
are 400 feet high.
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Niagara Falls would fit
just within the alcove here.
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There's a similar-sized alcove
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00:08:59,851 --> 00:09:01,951
and there's an even bigger one
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00:09:01,984 --> 00:09:04,418
that extends many miles
to the east.
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00:09:07,251 --> 00:09:11,551
These are the most extensive
falls, wet or dry, known today.
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00:09:11,584 --> 00:09:16,118
And in a valley below,
Vic finds another clue
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that vast amounts of water
once flowed here.
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00:09:21,551 --> 00:09:24,784
Features that look
like sinkholes, or potholes,
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00:09:24,818 --> 00:09:28,318
often found along the bottom
of turbulent rivers.
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00:09:28,351 --> 00:09:34,184
But these potholes
are super-sized.
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You can see it's maybe
50 feet deep or so.
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Potholes you see
in a normal river
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are about the size of a person.
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00:09:49,918 --> 00:09:52,084
Whereas this would hold
multiple elephants.
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00:09:54,751 --> 00:09:58,418
To Vic, all the evidence
points to flowing water
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on a massive scale.
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00:10:01,718 --> 00:10:04,684
But there is no water
flowing here now.
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00:10:08,618 --> 00:10:11,684
Today, the largest rivers
in the region
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are the Snake and the Columbia.
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00:10:17,551 --> 00:10:19,518
But could they
have played a role?
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00:10:19,551 --> 00:10:24,151
The Columbia River lies
about 30 miles to the north
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00:10:24,184 --> 00:10:26,851
and isn't big enough
to make this kind of feature.
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00:10:29,984 --> 00:10:32,151
The Snake River, as well,
is simply too small
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00:10:32,184 --> 00:10:36,818
to carve out potholes
and waterfalls on this scale.
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00:10:36,851 --> 00:10:40,951
To get such an immense volume
of water so fast,
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we need something spectacular
to happen.
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So where did the water
come from?
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To answer this question,
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scientists are looking
at a distant landscape
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3,500 miles away.
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Iceland.
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00:11:02,418 --> 00:11:06,951
This island, on the edge
of the Arctic Circle,
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00:11:06,984 --> 00:11:10,451
is a land of fire and ice.
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In its northeast corner
are scars on the landscape
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that bear a striking resemblance
to the Washington Scablands.
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Sheer cliffs over 300 feet high.
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A towering rock island.
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This is the AÁsbyrgi Canyon.
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And like scientists
in the Scablands,
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geomorphologist Mikael Attal
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00:11:56,151 --> 00:12:00,151
wants to understand
how it was created.
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00:12:03,551 --> 00:12:05,013
This really looks
like a dry waterfall.
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It's as if
there was a big waterfall here
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and it's not there anymore.
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00:12:14,418 --> 00:12:18,451
Its so similar to the Dry Falls
in the Scablands,
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00:12:18,484 --> 00:12:23,251
Mikael suspects they were formed
in the same way.
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00:12:27,384 --> 00:12:30,584
But how long did it take?
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00:12:36,951 --> 00:12:41,651
To find out, he's using
a relatively new technique
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00:12:41,684 --> 00:12:44,451
called surface-exposure dating.
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00:12:46,984 --> 00:12:49,651
Earth's surface
is constantly bombarded
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by cosmic rays from outer space.
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00:12:53,084 --> 00:12:58,151
Rocks buried in the Earth
are sheltered from these rays,
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00:12:58,184 --> 00:13:01,784
but as soon as the rocks are
exposed, like in these cliffs,
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00:13:01,818 --> 00:13:04,784
cosmic rays collide with atoms
at their surface.
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00:13:06,951 --> 00:13:08,651
The force of these collisions
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00:13:08,684 --> 00:13:11,751
knocks neutrons and protons
out of the atoms
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00:13:11,784 --> 00:13:14,618
and changes the elements
in the rocks.
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00:13:16,884 --> 00:13:19,451
This leads to the formation
of new elements,
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including a rare form of helium.
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00:13:23,551 --> 00:13:26,584
These rare helium atoms
build up over time,
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at a predictable rate,
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so by measuring
their concentration,
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00:13:31,884 --> 00:13:33,851
it's possible to determine
how long
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00:13:33,884 --> 00:13:36,384
the rock has been exposed.
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00:13:36,418 --> 00:13:38,451
It's like starting
the stopwatch.
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Mikael samples rocks
from all over the AÁsbyrgi Canyon
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00:13:46,918 --> 00:13:51,784
and compares the dates
when they were exposed.
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00:13:51,818 --> 00:13:54,418
His results revealed
something surprising.
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All the rocks in this area
were exposed at the same time,
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00:14:00,218 --> 00:14:07,151
meaning that this entire canyon
was carved out all at once.
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00:14:07,184 --> 00:14:10,018
This canyon was created
in one event
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00:14:10,051 --> 00:14:11,251
9,000 years ago.
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00:14:13,051 --> 00:14:16,451
A slow-moving force,
like a glacier, erosion,
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00:14:16,484 --> 00:14:18,984
or gradual uplift,
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00:14:19,018 --> 00:14:21,051
would have exposed the rocks
along the canyon
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00:14:21,084 --> 00:14:22,851
at different times.
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00:14:24,784 --> 00:14:29,184
So it had to be a fast-paced
natural disaster,
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00:14:29,218 --> 00:14:30,684
like a titanic flood.
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00:14:33,584 --> 00:14:36,184
It would have been a flood
on a scale
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00:14:36,218 --> 00:14:37,751
far greater than anything
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00:14:37,784 --> 00:14:40,084
that we have witnessed
in human history.
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00:14:45,018 --> 00:14:48,551
Thousands of miles away,
scientists in the Scablands
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00:14:48,584 --> 00:14:51,418
had zeroed in on the same idea.
219
00:14:53,218 --> 00:14:57,484
Everything in this landscape
was screaming,
220
00:14:57,518 --> 00:15:00,084
in terms of its signs or clues,
221
00:15:00,118 --> 00:15:02,518
that this was made
by catastrophic flooding.
222
00:15:04,984 --> 00:15:08,151
A flood big enough to carve
these vast landscapes
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00:15:08,184 --> 00:15:11,684
seems impossible,
224
00:15:11,718 --> 00:15:15,951
but flowing water can be
surprisingly powerful.
225
00:15:17,984 --> 00:15:20,151
The physical impact of a flood
226
00:15:20,184 --> 00:15:25,118
rises with every increase
in volume, speed, or duration,
227
00:15:25,151 --> 00:15:28,051
and it doesn't take a lot
to pack a punch.
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00:15:28,084 --> 00:15:30,851
A flood just six inches deep
229
00:15:30,884 --> 00:15:33,151
can knock people
right off their feet,
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00:15:33,184 --> 00:15:36,551
and a flow of just
seven miles an hour
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00:15:36,584 --> 00:15:40,351
can have the same force
as a tornado.
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00:15:45,784 --> 00:15:50,584
But is it possible to blast
through solid rock?
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00:15:50,618 --> 00:15:55,718
Could a flood carve out
enormous features like these?
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00:16:03,551 --> 00:16:05,151
The most destructive floods,
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00:16:05,184 --> 00:16:09,684
from Hurricane Katrina in 2005
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00:16:09,718 --> 00:16:14,084
to floods in Colorado in 2013,
237
00:16:14,118 --> 00:16:16,451
have etched painful memories.
238
00:16:16,484 --> 00:16:19,451
But they have done little
to make a mark
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00:16:19,484 --> 00:16:21,284
on the underlying bedrock...
240
00:16:24,618 --> 00:16:29,618
Like we see here in Iceland
and the Scablands.
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00:16:29,651 --> 00:16:31,384
No one has ever witnessed
anything
242
00:16:31,418 --> 00:16:33,084
even close in scale.
243
00:16:35,418 --> 00:16:37,718
We're talking about floods here
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00:16:37,751 --> 00:16:39,884
that completely changed the face
of the planet.
245
00:16:42,351 --> 00:16:43,684
Floods powerful enough
246
00:16:43,718 --> 00:16:46,384
to carve whole canyons
out of bedrock
247
00:16:46,418 --> 00:16:48,451
are rarely seen.
248
00:16:50,484 --> 00:16:56,018
But in 2002, one was finally
caught on camera.
249
00:16:57,151 --> 00:16:58,984
On July 4,
250
00:16:59,018 --> 00:17:01,051
after a severe storm
struck central Texas,
251
00:17:01,084 --> 00:17:06,451
Canyon Lake Reservoir flooded,
overtopping its dam.
252
00:17:06,484 --> 00:17:07,584
At its peak,
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00:17:07,618 --> 00:17:10,451
enough water to fill
an Olympic swimming pool
254
00:17:10,484 --> 00:17:14,184
poured over every two seconds.
255
00:17:14,218 --> 00:17:15,984
And when the floodwaters
subsided,
256
00:17:16,018 --> 00:17:19,351
they revealed a brand-new gorge,
257
00:17:19,384 --> 00:17:22,384
carved into the rock
23 feet deep
258
00:17:22,418 --> 00:17:24,918
and more than a mile long.
259
00:17:24,951 --> 00:17:28,118
This provided proof that floods
260
00:17:28,151 --> 00:17:33,718
can transform whole landscapes
in a matter of days,
261
00:17:33,751 --> 00:17:36,384
as long as there's enough water
flowing quickly enough
262
00:17:36,418 --> 00:17:39,418
to produce the necessary force.
263
00:17:44,518 --> 00:17:45,884
And in Iceland,
264
00:17:45,918 --> 00:17:50,284
geophysicist Magnús Gudmundsson
thinks he's figured out
265
00:17:50,318 --> 00:17:53,384
how such a massive release
of water could occur.
266
00:17:56,318 --> 00:17:59,118
He's come a hundred miles
south of AÁsbyrgi
267
00:17:59,151 --> 00:18:02,051
to the Vatnajoökull ice cap,
268
00:18:02,084 --> 00:18:07,451
the largest glacier in Europe,
269
00:18:07,484 --> 00:18:11,351
similar to glaciers
at the end of the last ice age,
270
00:18:11,384 --> 00:18:14,918
9,000 years ago.
271
00:18:14,951 --> 00:18:19,718
This glacier is the only
possible source of water
272
00:18:19,751 --> 00:18:22,651
to create these floods
that made AÁsbyrgi.
273
00:18:24,951 --> 00:18:28,284
In places, it's 3,000 feet deep,
274
00:18:28,318 --> 00:18:33,018
hundreds of cubic miles of water
locked up as ice.
275
00:18:33,051 --> 00:18:34,584
We have all this ice here,
276
00:18:34,618 --> 00:18:37,618
but how does it become a flood?
277
00:18:37,651 --> 00:18:41,018
Magnús believes the secret lies
278
00:18:41,051 --> 00:18:44,384
in what's hidden
beneath the ice cap:
279
00:18:44,418 --> 00:18:48,484
seven huge volcanoes.
280
00:18:48,518 --> 00:18:53,051
In 1996, one of these,
the volcano called Grímsvoötn,
281
00:18:53,084 --> 00:18:56,651
erupted, triggering the most
catastrophic flood in Iceland
282
00:18:56,684 --> 00:18:58,584
for nearly a century.
283
00:18:58,618 --> 00:19:01,584
From time to time, we have
these very large eruptions
284
00:19:01,618 --> 00:19:06,684
that melt enormous amounts
of ice in a matter of hours.
285
00:19:10,151 --> 00:19:13,384
Almost a cubic mile
of meltwater from the eruption
286
00:19:13,418 --> 00:19:15,884
tore across the landscape
287
00:19:15,918 --> 00:19:19,151
at 16 times the rate
of Niagara Falls,
288
00:19:19,184 --> 00:19:23,851
destroying roads, bridges,
and power lines.
289
00:19:28,151 --> 00:19:32,818
So today, Magnús is monitoring
the volcano.
290
00:19:36,884 --> 00:19:40,084
By recording elevation
and movement,
291
00:19:40,118 --> 00:19:43,084
his team has discovered
there is a lake of meltwater
292
00:19:43,118 --> 00:19:45,451
beneath the ice.
293
00:19:45,484 --> 00:19:50,051
You're actually standing on an
ice shelf floating on the lake.
294
00:19:55,984 --> 00:19:58,884
Magnús now believes
that 9,000 years ago...
295
00:20:01,284 --> 00:20:03,784
...a giant eruption
under the ice cap
296
00:20:03,818 --> 00:20:06,851
unleashed a colossal flood
297
00:20:06,884 --> 00:20:10,351
powerful enough
to carve the AÁsbyrgi Canyon.
298
00:20:13,951 --> 00:20:18,584
So could a chain of events
like this have also triggered
299
00:20:18,618 --> 00:20:22,484
a flood massive enough to carve
the Scablands in North America?
300
00:20:30,418 --> 00:20:33,484
Just a hundred miles
west of the Scablands
301
00:20:33,518 --> 00:20:35,718
are many active volcanoes,
302
00:20:35,751 --> 00:20:40,851
notably Mount St. Helens,
partially covered in ice.
303
00:20:47,118 --> 00:20:51,351
In 1980,
Mount St. Helens erupted,
304
00:20:51,384 --> 00:20:56,251
releasing an enormous amount
of heat,
305
00:20:56,284 --> 00:20:58,884
enough to melt the ice
around its crater
306
00:20:58,918 --> 00:21:01,284
and trigger dramatic floods.
307
00:21:05,918 --> 00:21:10,318
But even if all the ice on this
volcano had suddenly melted...
308
00:21:13,051 --> 00:21:15,884
It would not be enough
309
00:21:15,918 --> 00:21:21,818
to carve out rock
over a 16,000-square-mile area.
310
00:21:29,784 --> 00:21:33,418
But what about in the past?
311
00:21:33,451 --> 00:21:37,318
To find out if ancient ice
were to blame,
312
00:21:37,351 --> 00:21:43,151
Vic Baker needs to find out
when the landscape was created
313
00:21:43,184 --> 00:21:46,418
with the technique
of surface-exposure dating
314
00:21:46,451 --> 00:21:47,384
used in Iceland.
315
00:21:50,384 --> 00:21:53,818
Taking samples from all over
the Scablands,
316
00:21:53,851 --> 00:21:55,184
he discovers most of the rocks
317
00:21:55,218 --> 00:22:00,918
were exposed within a few
thousand years of each other,
318
00:22:00,951 --> 00:22:04,984
and one date in particular
stands out.
319
00:22:06,984 --> 00:22:08,784
Many of the dates we get
320
00:22:08,818 --> 00:22:11,151
are in the range
of about 16,000 years ago.
321
00:22:12,751 --> 00:22:15,451
Although ice sheets covered
much of North America
322
00:22:15,484 --> 00:22:18,851
16,000 years ago,
323
00:22:18,884 --> 00:22:20,884
geologists believe
the ice stopped short
324
00:22:20,918 --> 00:22:24,418
of the volcanoes
of Washington state.
325
00:22:27,318 --> 00:22:30,284
The trail of clues
seemed to dry up...
326
00:22:35,284 --> 00:22:39,251
...until a surprising discovery
327
00:22:39,284 --> 00:22:44,584
200 miles east of the Scablands,
in Missoula, Montana.
328
00:22:46,851 --> 00:22:51,718
Here, geologist Larry Smith
is heading up into the hills,
329
00:22:51,751 --> 00:22:55,584
where he sees a series
of horizontal lines
330
00:22:55,618 --> 00:23:01,518
a thousand feet
above the valley floor.
331
00:23:01,551 --> 00:23:03,218
They look very much
332
00:23:03,251 --> 00:23:06,318
like they'd have been cut
into the hillside
333
00:23:06,351 --> 00:23:09,884
by waves beating
against the rock.
334
00:23:09,918 --> 00:23:14,584
These lines are clearly
lake shorelines,
335
00:23:14,618 --> 00:23:20,318
and show that an immense
body of water temporarily filled
336
00:23:20,351 --> 00:23:22,818
these now-dry valleys
of western Montana.
337
00:23:25,651 --> 00:23:30,118
Tracing these ancient shorelines
for hundreds of miles,
338
00:23:30,151 --> 00:23:33,918
geologists have calculated that
these valleys were once filled
339
00:23:33,951 --> 00:23:37,684
by a body of water
larger than Lake Ontario.
340
00:23:41,084 --> 00:23:43,284
When there was a lake here
at 4,200 feet,
341
00:23:43,318 --> 00:23:46,551
we would have had a beach
right here in front of us,
342
00:23:46,584 --> 00:23:48,518
or a shoreline,
343
00:23:48,551 --> 00:23:52,918
and extending all the way across
to the other side of the valley,
344
00:23:52,951 --> 00:23:55,284
with a thousand feet of water
345
00:23:55,318 --> 00:23:58,451
over what is now
the city of Missoula.
346
00:23:58,484 --> 00:24:03,384
Geologists call it
Glacial Lake Missoula.
347
00:24:03,418 --> 00:24:07,684
And despite being 200 miles away
from the Scablands,
348
00:24:07,718 --> 00:24:09,818
Larry suspects it held
enough water
349
00:24:09,851 --> 00:24:11,918
to tear through the area.
350
00:24:14,918 --> 00:24:18,218
That is a vast amount of water,
351
00:24:18,251 --> 00:24:22,251
and if this lake drained
very rapidly,
352
00:24:22,284 --> 00:24:25,518
it would be fundamental
353
00:24:25,551 --> 00:24:27,951
to carving the channel
in Scabland.
354
00:24:29,818 --> 00:24:32,018
But today, there is no lake here
355
00:24:32,051 --> 00:24:34,918
because the valley
is open-ended.
356
00:24:36,984 --> 00:24:41,918
So where are the formations
that held the water in place?
357
00:24:44,284 --> 00:24:45,718
So the question is, is
358
00:24:45,751 --> 00:24:50,251
where, when, and how did
a dam form to create this lake?
359
00:24:52,118 --> 00:24:53,618
Searching for clues,
360
00:24:53,651 --> 00:24:56,751
Larry travels back down
to Clark Fork,
361
00:24:56,784 --> 00:25:00,784
at the narrow end of the valley.
362
00:25:00,818 --> 00:25:03,518
He sees no signs of landslides
or rock falls
363
00:25:03,551 --> 00:25:06,151
that could have dammed the lake
in the past.
364
00:25:06,184 --> 00:25:11,984
But on the bare rocks, he spots
some tell-tale markings.
365
00:25:12,018 --> 00:25:15,651
You see scratches
within the rock,
366
00:25:15,684 --> 00:25:19,418
geologically it is impossible
367
00:25:19,451 --> 00:25:24,284
to smooth off rock and
scratch it without glacial ice.
368
00:25:25,784 --> 00:25:28,751
As the glacier moves,
rocks embedded within it
369
00:25:28,784 --> 00:25:32,551
scratch the bedrock
like sandpaper.
370
00:25:32,584 --> 00:25:36,051
So these scratches are evidence
that during the last ice age,
371
00:25:36,084 --> 00:25:39,984
a glacier moved
across this valley.
372
00:25:40,018 --> 00:25:42,118
And by mapping where rocks
have been scratched,
373
00:25:42,151 --> 00:25:46,451
geologists have discovered
that the Clark Fork River Valley
374
00:25:46,484 --> 00:25:49,251
was once blocked
by a giant finger of ice
375
00:25:49,284 --> 00:25:53,918
23 miles wide
and half a mile deep.
376
00:25:53,951 --> 00:25:58,284
Larry Smith believes this
ice dam created Lake Missoula.
377
00:25:58,318 --> 00:26:02,584
It blocked the drainage
of the Clark Fork River.
378
00:26:02,618 --> 00:26:04,784
The water had nowhere else
to go,
379
00:26:04,818 --> 00:26:09,418
so backed up a lake behind this
large glacier in this valley.
380
00:26:12,618 --> 00:26:17,851
All the evidence points
to a massive reservoir of water
381
00:26:17,884 --> 00:26:23,118
held in place
by a giant dam of ice,
382
00:26:23,151 --> 00:26:24,384
a lake large enough
383
00:26:24,418 --> 00:26:27,551
to have carved out the canyons
of the Scablands...
384
00:26:30,618 --> 00:26:34,618
...if it were released
in one catastrophic event.
385
00:26:46,218 --> 00:26:48,584
The idea that there
had once been a lake here
386
00:26:48,618 --> 00:26:50,384
that had suddenly drained
387
00:26:50,418 --> 00:26:52,851
also explains one of the other
striking features
388
00:26:52,884 --> 00:26:58,884
of the valley floor...
Giant ripples.
389
00:27:03,484 --> 00:27:07,084
These straight crested hills
are current ripples
390
00:27:07,118 --> 00:27:10,718
that show water flowing
from where we're standing
391
00:27:10,751 --> 00:27:12,151
off to the distance.
392
00:27:14,884 --> 00:27:18,018
Ripples like this
are made by flowing water,
393
00:27:18,051 --> 00:27:22,284
like the tide moving in and out
on a beach.
394
00:27:22,318 --> 00:27:23,551
The faster the flow of water,
395
00:27:23,584 --> 00:27:26,118
the larger
and more widely spaced
396
00:27:26,151 --> 00:27:28,084
the ripples become.
397
00:27:28,118 --> 00:27:30,118
Here, they're giant things
398
00:27:30,151 --> 00:27:33,118
that are spaced
hundreds of feet apart
399
00:27:33,151 --> 00:27:36,784
and they're tens of feet high.
400
00:27:36,818 --> 00:27:41,751
These ripples are so high,
the lake water that created them
401
00:27:41,784 --> 00:27:43,044
must have poured
through this valley
402
00:27:43,051 --> 00:27:46,318
at speeds of up to 80 miles
an hour.
403
00:27:51,884 --> 00:27:53,718
It's evidence that Lake Missoula
404
00:27:53,751 --> 00:27:57,251
was unleashed rapidly
in a massive flood.
405
00:27:59,418 --> 00:28:04,551
But that means the 23-mile-wide
ice dam holding it in place
406
00:28:04,584 --> 00:28:07,718
must have suddenly given way.
407
00:28:10,684 --> 00:28:13,018
How could an ice dam
of this scale
408
00:28:13,051 --> 00:28:17,651
fail so catastrophically?
409
00:28:22,718 --> 00:28:25,584
The exposure dates of the rocks
in the Scablands
410
00:28:25,618 --> 00:28:28,751
reveal the flood occurred well
before the end of the Ice Age.
411
00:28:28,784 --> 00:28:33,651
This rules out gradual melting
from a warming climate.
412
00:28:35,418 --> 00:28:38,618
Could looking at modern
dam failures hold a clue?
413
00:28:41,151 --> 00:28:44,351
You people down the stream
better get out.
414
00:28:44,384 --> 00:28:46,351
In 1976,
415
00:28:46,384 --> 00:28:50,184
the newly constructed
300-foot-high Teton Dam,
416
00:28:50,218 --> 00:28:53,218
in Idaho, failed,
417
00:28:53,251 --> 00:28:56,484
unleashing almost
80 billion gallons of water.
418
00:29:00,484 --> 00:29:02,651
Investigators discovered
419
00:29:02,684 --> 00:29:05,084
that water had seeped
under the earth-filled dam,
420
00:29:05,118 --> 00:29:08,584
eroding it from below.
421
00:29:08,618 --> 00:29:11,984
Larry Smith believes water
seeping under the ice dam
422
00:29:12,018 --> 00:29:15,318
also caused the catastrophic
release of Lake Missoula.
423
00:29:18,184 --> 00:29:23,018
At the bottom
of this 2,000-foot-deep lake,
424
00:29:23,051 --> 00:29:25,618
the water pressures are immense,
425
00:29:25,651 --> 00:29:31,151
and any small cracks in the ice
will get penetrated
426
00:29:31,184 --> 00:29:32,818
by that high-pressure water.
427
00:29:32,851 --> 00:29:37,484
In doing so, that'll expand
that crack network
428
00:29:37,518 --> 00:29:41,118
to form tunnels under the ice.
429
00:29:45,051 --> 00:29:47,451
Lake water began draining
through these tunnels
430
00:29:47,484 --> 00:29:50,784
at a faster and faster rate...
431
00:29:53,084 --> 00:29:58,484
...until the whole ice dam
suddenly collapsed.
432
00:30:09,384 --> 00:30:13,218
It falls within minutes
to hours,
433
00:30:13,251 --> 00:30:15,618
with a cascade of water
coming through the area.
434
00:30:20,118 --> 00:30:22,384
All signs point to Lake Missoula
435
00:30:22,418 --> 00:30:25,618
being the source
of a catastrophic flood.
436
00:30:27,484 --> 00:30:30,384
Still, how likely is it
that floodwater could travel
437
00:30:30,418 --> 00:30:33,784
hundreds of miles southwest
to the Scablands
438
00:30:33,818 --> 00:30:36,218
with enough power to carve out
solid rock
439
00:30:36,251 --> 00:30:39,518
and transform
the entire landscape?
440
00:30:50,251 --> 00:30:55,351
Roger Denlinger studies
fluid dynamics.
441
00:30:55,384 --> 00:30:59,618
He's taken the volume
of ancient Lake Missoula
442
00:30:59,651 --> 00:31:02,251
and 3D maps of the Scablands
443
00:31:02,284 --> 00:31:05,684
to build a computer model
444
00:31:05,718 --> 00:31:07,718
that will predict
where the ancient flood
445
00:31:07,751 --> 00:31:11,151
would have traveled.
446
00:31:11,184 --> 00:31:15,351
Effectively, you're just pouring
water over the landscape.
447
00:31:15,384 --> 00:31:18,184
This is simply water flowing
over the Earth's surface,
448
00:31:18,218 --> 00:31:20,918
and it's going to always head
in the direction
449
00:31:20,951 --> 00:31:22,151
that it sees as downhill.
450
00:31:24,284 --> 00:31:26,651
Roger's model
will also determine
451
00:31:26,684 --> 00:31:30,384
the depth of the water.
452
00:31:30,418 --> 00:31:33,451
And this color bar shows
the flood's erosive power.
453
00:31:35,851 --> 00:31:38,651
If flow lines in the model
turn red,
454
00:31:38,684 --> 00:31:41,684
Roger knows the water
was flowing with enough force
455
00:31:41,718 --> 00:31:43,218
to carve solid rock.
456
00:31:43,251 --> 00:31:46,151
At this point,
we're going to break the dam.
457
00:31:59,551 --> 00:32:02,018
The moment it's released
from the ice dam,
458
00:32:02,051 --> 00:32:04,418
the lake water rushes southwest,
459
00:32:04,451 --> 00:32:06,051
toward what we know today
460
00:32:06,084 --> 00:32:08,984
as the eroded landscape
of the Scablands.
461
00:32:11,918 --> 00:32:13,751
And not only that,
462
00:32:13,784 --> 00:32:16,818
the places that the model
has highlighted in red,
463
00:32:16,851 --> 00:32:19,251
where the power of the flood
is greatest,
464
00:32:19,284 --> 00:32:22,151
exactly match the location
465
00:32:22,184 --> 00:32:25,651
of the most dramatically
transformed landscapes today:
466
00:32:28,084 --> 00:32:30,084
The Dry Falls;
467
00:32:32,684 --> 00:32:35,418
The rock islands,
468
00:32:38,251 --> 00:32:40,884
And the sheer gorges.
469
00:32:46,718 --> 00:32:50,351
We get damage to the surface
in exactly the areas
470
00:32:50,384 --> 00:32:51,784
that we see today.
471
00:32:53,684 --> 00:32:56,818
Roger's model
supports the theory
472
00:32:56,851 --> 00:32:59,551
that a giant flood
from Lake Missoula
473
00:32:59,584 --> 00:33:02,184
carved this landscape,
474
00:33:02,218 --> 00:33:07,784
and reveals that the waters
reached unimaginable heights.
475
00:33:07,818 --> 00:33:10,518
This water came up 800 feet...
That's huge.
476
00:33:12,651 --> 00:33:16,618
Most people think of floods
by watching the TV
477
00:33:16,651 --> 00:33:18,551
and they see the water rising
in a river,
478
00:33:18,584 --> 00:33:20,784
and they see a house
going underwater,
479
00:33:20,818 --> 00:33:24,051
maybe there's a person
on top of the house.
480
00:33:24,084 --> 00:33:28,151
Think of water hundreds of feet
above the house,
481
00:33:28,184 --> 00:33:32,951
that's the difference
in the scale of this flooding.
482
00:33:32,984 --> 00:33:35,484
Bringing all the evidence
together,
483
00:33:35,518 --> 00:33:39,684
scientists can now unpack
the catastrophic flood
484
00:33:39,718 --> 00:33:41,018
blow by blow.
485
00:33:41,051 --> 00:33:45,051
Around 16,000 years ago,
486
00:33:45,084 --> 00:33:49,084
the vast ice dam holding back
Lake Missoula failed,
487
00:33:49,118 --> 00:33:53,451
suddenly unleashing
500 cubic miles of water.
488
00:33:59,418 --> 00:34:01,651
It was equivalent in volume
489
00:34:01,684 --> 00:34:05,918
to ten times all the rivers
of the world's natural flow.
490
00:34:05,951 --> 00:34:11,584
The raging torrent
tears across Washington state,
491
00:34:11,618 --> 00:34:13,318
ripping out billions
of tons of rock
492
00:34:13,351 --> 00:34:16,884
from the once-flat landscape.
493
00:34:16,917 --> 00:34:19,651
There would be blocks of ice,
494
00:34:19,684 --> 00:34:21,784
there would be boulders,
there'd be roiling water,
495
00:34:21,818 --> 00:34:25,051
the sound would be overwhelming.
496
00:34:25,084 --> 00:34:27,684
In a matter of hours,
497
00:34:27,718 --> 00:34:29,718
the flood reaches
the Pacific Ocean,
498
00:34:29,751 --> 00:34:35,251
carrying with it 1,200
cubic miles of rock and earth,
499
00:34:35,284 --> 00:34:38,184
violently torn
from the Scablands.
500
00:34:42,084 --> 00:34:45,084
To do all this landscape change
501
00:34:45,118 --> 00:34:46,818
within a few days to a few weeks
502
00:34:46,851 --> 00:34:48,318
is just mind-expanding.
503
00:34:52,951 --> 00:34:56,318
Even Hollywood disaster movies
do not compare
504
00:34:56,351 --> 00:34:57,418
to what would have happened
505
00:34:57,451 --> 00:35:00,118
as this flood came across
the landscape.
506
00:35:06,718 --> 00:35:11,151
This dramatic event entirely
reshaped the landscape.
507
00:35:11,184 --> 00:35:14,484
But there is one final twist
in the tale.
508
00:35:15,751 --> 00:35:18,784
Further research,
based on core samples
509
00:35:18,818 --> 00:35:21,618
drilled out from the floor
of the Pacific Ocean,
510
00:35:21,651 --> 00:35:25,484
suggests that the Scablands
are a product not of one,
511
00:35:25,518 --> 00:35:29,018
but of many floods.
512
00:35:29,051 --> 00:35:32,151
The evidence reveals
that during the Ice Age,
513
00:35:32,184 --> 00:35:34,884
beginning around
20,000 years ago,
514
00:35:34,918 --> 00:35:39,051
repeated floods
tore across the landscape...
515
00:35:41,151 --> 00:35:46,584
...as the giant ice dam
repeatedly broke,
516
00:35:46,618 --> 00:35:51,718
reformed, and then broke again.
517
00:35:52,984 --> 00:35:55,084
The circumstances that created
518
00:35:55,118 --> 00:35:56,851
this immense volume of water
519
00:35:56,884 --> 00:35:59,284
produced multiple floods.
520
00:36:06,518 --> 00:36:08,651
Decades of geological
detective work
521
00:36:08,684 --> 00:36:12,184
show that the scarred
and eroded landscapes
522
00:36:12,218 --> 00:36:16,584
of Washington state,
as well as Iceland,
523
00:36:16,618 --> 00:36:20,618
both bear the fingerprints
of mega-floods.
524
00:36:20,651 --> 00:36:25,851
And now, this discovery
is helping scientists unravel
525
00:36:25,884 --> 00:36:29,518
a mystery in another part
of the world.
526
00:36:33,184 --> 00:36:36,718
Thousands of miles away
is the channel that separates
527
00:36:36,751 --> 00:36:39,684
what is now England from France.
528
00:36:41,784 --> 00:36:44,484
Today, it links the North Sea
in the east
529
00:36:44,518 --> 00:36:47,518
to the Atlantic Ocean
in the west.
530
00:36:47,551 --> 00:36:49,584
Called the English Channel,
531
00:36:49,618 --> 00:36:53,284
it's the busiest shipping lane
in the world.
532
00:36:57,818 --> 00:37:01,451
And towering more than
350 feet above it,
533
00:37:01,484 --> 00:37:03,251
on the south coast of England,
534
00:37:03,284 --> 00:37:06,884
are the White Cliffs of Dover.
535
00:37:08,451 --> 00:37:10,251
Geologists, like James Lawrence,
536
00:37:10,284 --> 00:37:14,184
now think
these iconic chalk cliffs
537
00:37:14,218 --> 00:37:16,651
hold an extraordinary secret,
538
00:37:16,684 --> 00:37:20,384
and he's going over the edge
to hunt for the evidence.
539
00:37:23,184 --> 00:37:27,051
Because these cliffs
look almost identical to cliffs
540
00:37:27,084 --> 00:37:29,118
on the other side
of the channel,
541
00:37:29,151 --> 00:37:32,251
on the northern coast of France.
542
00:37:33,584 --> 00:37:35,818
People don't realize
543
00:37:35,851 --> 00:37:37,718
that if I was to go over
to France,
544
00:37:37,751 --> 00:37:42,018
we could find similar
chalk cliffs.
545
00:37:42,051 --> 00:37:46,051
This chalk formed
100 million years ago,
546
00:37:46,084 --> 00:37:49,818
when this whole area
was covered by a tropical sea.
547
00:37:52,084 --> 00:37:55,518
The ancient sea teemed
with microscopic organisms.
548
00:37:55,551 --> 00:37:58,818
When they died,
their calcium-rich skeletons
549
00:37:58,851 --> 00:38:00,951
fell to the sea bed.
550
00:38:00,984 --> 00:38:04,751
Over time,
these built up in thick layers
551
00:38:04,784 --> 00:38:07,184
and were compressed into chalk,
552
00:38:07,218 --> 00:38:08,784
a kind of limestone.
553
00:38:08,818 --> 00:38:13,218
We are getting
exactly the same rocks
554
00:38:13,251 --> 00:38:16,251
which have been deposited
in exactly the same environment
555
00:38:16,284 --> 00:38:18,318
on this side of the channel,
556
00:38:18,351 --> 00:38:21,184
and on the French side
of the channel.
557
00:38:25,751 --> 00:38:27,918
And James is discovering
558
00:38:27,951 --> 00:38:30,951
that the connection between
the cliffs in France and England
559
00:38:30,984 --> 00:38:33,818
goes beyond the chalk itself.
560
00:38:41,118 --> 00:38:45,018
Embedded in the white chalk
are a series of horizontal bands
561
00:38:45,051 --> 00:38:49,084
of a dark rock called flint.
562
00:38:49,118 --> 00:38:54,384
Here I have a fantastic band
of flint.
563
00:38:56,318 --> 00:38:59,018
Flint, a form of the
mineral quartz,
564
00:38:59,051 --> 00:39:02,584
is formed by changes
in ocean chemistry.
565
00:39:07,451 --> 00:39:10,384
But these changes occur
only occasionally,
566
00:39:10,418 --> 00:39:13,951
resulting in these
distinctive dark bands.
567
00:39:16,418 --> 00:39:17,618
These flint bands
568
00:39:17,651 --> 00:39:20,151
are continuous
throughout the chalk.
569
00:39:22,918 --> 00:39:26,684
This band of flint
runs through the entire cliff,
570
00:39:26,718 --> 00:39:31,418
and there are dozens
running horizontally...
571
00:39:34,818 --> 00:39:38,618
...each one at a different level
in the chalk.
572
00:39:38,651 --> 00:39:43,684
Taken together, these parallel
bands of dark flint
573
00:39:43,718 --> 00:39:47,218
form a unique geological
fingerprint in the white cliff.
574
00:39:50,251 --> 00:39:54,484
What's extraordinary is that
the same geological fingerprint
575
00:39:54,518 --> 00:39:57,618
is visible on the other side
of the channel.
576
00:40:00,918 --> 00:40:03,884
So the chalk and the flint
in these cliffs forms a bar code
577
00:40:03,918 --> 00:40:07,718
and is exactly the same
as the chalk and the flint
578
00:40:07,751 --> 00:40:09,618
in the cliffs in France.
579
00:40:09,651 --> 00:40:15,818
The spacing and levels of the
flint layers perfectly align.
580
00:40:18,651 --> 00:40:23,618
To James Lawrence, this raises
an extraordinary possibility.
581
00:40:23,651 --> 00:40:27,118
So what we know from this
evidence is that a chalk ridge
582
00:40:27,151 --> 00:40:30,451
once connected England
and France.
583
00:40:30,484 --> 00:40:33,651
These flint layers tell us
584
00:40:33,684 --> 00:40:35,918
that hundreds of thousands
of years ago,
585
00:40:35,951 --> 00:40:38,518
a ridge of chalk
almost seven miles wide
586
00:40:38,551 --> 00:40:42,618
once extended 21 miles
across the channel,
587
00:40:42,651 --> 00:40:47,551
joining what is now Britain
to the European continent.
588
00:40:49,684 --> 00:40:50,918
So it's quite incredible
589
00:40:50,951 --> 00:40:52,751
to think that there
would have been a land mass
590
00:40:52,784 --> 00:40:54,784
stretching across the sea.
591
00:40:59,451 --> 00:41:04,384
But this discovery
raises a brand-new mystery.
592
00:41:06,151 --> 00:41:08,118
Somehow, the cliffs
between England and France
593
00:41:08,151 --> 00:41:10,584
have been separated over time.
594
00:41:12,484 --> 00:41:15,084
If Britain and France
were once joined,
595
00:41:15,118 --> 00:41:20,284
what force separated them and
turned Britain into an island?
596
00:41:24,618 --> 00:41:26,851
Control, Maverick.
597
00:41:28,218 --> 00:41:32,118
While exploring the sea bed
of the English Channel,
598
00:41:32,151 --> 00:41:35,084
geologist Jenny Collier
finds a telling clue.
599
00:41:39,618 --> 00:41:43,618
Four, five, six... Ten
meters in a split second.
600
00:41:44,984 --> 00:41:47,718
We've got a really steep
drop-off in the topography
601
00:41:47,751 --> 00:41:51,084
and it's the edge
of a really unusual landform.
602
00:41:51,118 --> 00:41:55,618
Using sonar to measure
the depth of the channel,
603
00:41:55,651 --> 00:41:59,651
Jenny is surprised to find what
appears to be a steep canyon
604
00:41:59,684 --> 00:42:03,318
carved into solid bedrock.
605
00:42:05,484 --> 00:42:10,084
Sonar works by firing
sound waves at the sea bed.
606
00:42:10,118 --> 00:42:12,751
The deeper the water,
the longer it takes the sound
607
00:42:12,784 --> 00:42:14,018
to make the round trip.
608
00:42:16,351 --> 00:42:19,084
Jenny expected the channel floor
to be flat,
609
00:42:19,118 --> 00:42:24,551
but the sonar has revealed
something far more dramatic.
610
00:42:24,584 --> 00:42:27,951
We've discovered
611
00:42:27,984 --> 00:42:29,951
just an extraordinary
geological event,
612
00:42:29,984 --> 00:42:31,484
right in the middle
of the straits.
613
00:42:33,251 --> 00:42:37,051
To learn more about
this major geological find,
614
00:42:37,084 --> 00:42:40,484
she and her colleagues
took on a massive task.
615
00:42:43,518 --> 00:42:46,984
Using a more advanced
sonar system,
616
00:42:47,018 --> 00:42:49,251
they are mapping
53 square miles of the channel,
617
00:42:49,284 --> 00:42:52,451
to an accuracy of four inches.
618
00:42:55,218 --> 00:42:57,951
What this reveals
is a strange picture
619
00:42:57,984 --> 00:43:01,484
of channels, rock islands,
and valleys,
620
00:43:01,518 --> 00:43:06,851
carved nearly 300 feet down,
into the rock of the sea bed.
621
00:43:10,918 --> 00:43:12,711
I mean, we haven't got anything
like this in Europe.
622
00:43:12,718 --> 00:43:17,618
There's really only one place
that has all of these features.
623
00:43:19,384 --> 00:43:22,618
Without the water, the landscape
beneath the English Channel
624
00:43:22,651 --> 00:43:26,984
appears to have steep valleys
and islands carved into it.
625
00:43:27,018 --> 00:43:31,251
It looks eerily similar
to the channeled Scablands
626
00:43:31,284 --> 00:43:34,884
of Washington state.
627
00:43:34,918 --> 00:43:36,884
But was this underwater
landscape
628
00:43:36,918 --> 00:43:41,984
also created by a mega-flood?
629
00:43:42,018 --> 00:43:44,684
The only way you can dig out
islands into solid bedrock
630
00:43:44,718 --> 00:43:47,484
is to have extreme water flows,
631
00:43:47,518 --> 00:43:49,384
and that basically pointed us
towards,
632
00:43:49,418 --> 00:43:52,518
this was yet another
catastrophic flood terrain.
633
00:43:54,418 --> 00:43:58,884
What the Scablands revealed is
that carving solid rock
634
00:43:58,918 --> 00:44:04,618
requires a huge reservoir
of water to be trapped,
635
00:44:04,651 --> 00:44:09,251
then released in a single
cataclysmic event.
636
00:44:13,018 --> 00:44:18,484
But today, the English Channel
flows between two open seas.
637
00:44:18,518 --> 00:44:22,584
So how could a large enough
volume of water
638
00:44:22,618 --> 00:44:24,784
have built up
to cause a mega-flood?
639
00:44:29,784 --> 00:44:33,784
Geologist Phil Gibbard
believes he has an answer.
640
00:44:33,818 --> 00:44:40,484
And the evidence lies 120 miles
north of the English Channel,
641
00:44:40,518 --> 00:44:42,818
on the coast of the North Sea,
642
00:44:42,851 --> 00:44:48,584
at the bottom of these cliffs.
643
00:44:48,618 --> 00:44:51,718
What we've got here
is a glacial deposit
644
00:44:51,751 --> 00:44:55,951
which is from about
450,000 years ago.
645
00:44:57,984 --> 00:45:00,851
Deep, fine-grained deposits
like this
646
00:45:00,884 --> 00:45:03,051
were laid down
across Northern Europe
647
00:45:03,084 --> 00:45:08,151
as giant ice sheets
ground over rocks.
648
00:45:08,184 --> 00:45:14,218
450,000 years ago, England
was in the grip of an ice age.
649
00:45:15,918 --> 00:45:20,384
Ice sheets, hundreds of miles
across and a mile high,
650
00:45:20,418 --> 00:45:23,251
reached down from Scandinavia.
651
00:45:23,284 --> 00:45:26,884
They would have dammed the
northern edge of the North Sea.
652
00:45:26,918 --> 00:45:29,584
To the south,
the intact ridge of chalk
653
00:45:29,618 --> 00:45:31,884
between what is now
France and England
654
00:45:31,918 --> 00:45:34,151
formed a natural dam.
655
00:45:34,184 --> 00:45:37,251
Phil believes that meltwater
656
00:45:37,284 --> 00:45:41,284
from the ice sheets and rivers
pouring into the North Sea
657
00:45:41,318 --> 00:45:43,184
had nowhere to go.
658
00:45:43,218 --> 00:45:47,284
A vast amount of water built up
behind the chalk ridge.
659
00:45:47,318 --> 00:45:51,284
He sees the evidence
for this ice age reservoir
660
00:45:51,318 --> 00:45:52,851
in the sea cliffs,
661
00:45:52,884 --> 00:45:56,884
as thin horizontal layers
of silt.
662
00:45:56,918 --> 00:45:59,218
The sediments are horizontal,
as you see.
663
00:45:59,251 --> 00:46:01,951
That horizontality
can only be produced
664
00:46:01,984 --> 00:46:05,151
in a lake situation,
a standing-water situation.
665
00:46:05,184 --> 00:46:10,351
And not in a turbulent area,
like an ocean.
666
00:46:10,384 --> 00:46:12,718
Phil has discovered
similar-looking formations
667
00:46:12,751 --> 00:46:15,084
in other places
around the North Sea,
668
00:46:15,118 --> 00:46:19,318
some a hundred feet
above sea level today.
669
00:46:21,951 --> 00:46:24,151
So this was a massive lake
on the scale
670
00:46:24,184 --> 00:46:26,884
of the Great Lakes
in North America,
671
00:46:26,918 --> 00:46:30,151
and this lake provides
the only possible source
672
00:46:30,184 --> 00:46:33,851
for the mega-flood
that formed the Dover Straits.
673
00:46:33,884 --> 00:46:39,051
Could this enormous
reservoir, a glacial lake,
674
00:46:39,084 --> 00:46:41,851
have suddenly drained
to form the dramatic features
675
00:46:41,884 --> 00:46:44,318
on the bed
of the English Channel?
676
00:46:44,351 --> 00:46:46,951
And if so, how?
677
00:46:50,018 --> 00:46:52,551
In order to carve
these features,
678
00:46:52,584 --> 00:46:54,751
this rock ridge must have failed
very, very rapidly.
679
00:46:56,751 --> 00:46:58,684
But what could have caused this?
680
00:47:03,251 --> 00:47:05,451
How could the giant ridge
of solid rock
681
00:47:05,484 --> 00:47:07,651
between France and Britain
682
00:47:07,684 --> 00:47:12,284
have given way
so catastrophically?
683
00:47:14,884 --> 00:47:18,951
A clue lies in the way chalk
reacts to water.
684
00:47:22,951 --> 00:47:26,318
Having a glacial lake
in contact with a chalk ridge
685
00:47:26,351 --> 00:47:29,984
would have saturated the chalk,
making it much weaker
686
00:47:30,018 --> 00:47:32,384
and much more likely to fail.
687
00:47:32,418 --> 00:47:36,684
When water soaks into chalk
and saturates it,
688
00:47:36,718 --> 00:47:41,051
the chalk can lose
half its strength,
689
00:47:41,084 --> 00:47:46,818
making it far more likely
to fail.
690
00:47:46,851 --> 00:47:48,644
One of the problems
with the chalk being so weak
691
00:47:48,651 --> 00:47:51,084
is that it will often lead
to cliff collapses,
692
00:47:51,118 --> 00:47:53,351
like the one we can see
behind us.
693
00:47:56,484 --> 00:48:00,784
Every year, thousands of tons
of rain and wave-soaked chalk
694
00:48:00,818 --> 00:48:04,051
collapse into the channel,
695
00:48:04,084 --> 00:48:08,851
dramatically eroding
the coastline.
696
00:48:14,284 --> 00:48:17,718
Many geologists now believe
that during a previous ice age
697
00:48:17,751 --> 00:48:20,184
almost a half million years ago,
698
00:48:20,218 --> 00:48:22,018
water from the North Sea
reservoir
699
00:48:22,051 --> 00:48:26,184
soaked the chalk ridge,
fatally weakening it.
700
00:48:28,918 --> 00:48:31,284
Once the lake was deep enough,
701
00:48:31,318 --> 00:48:37,318
water began pouring over the top
of the ridge in a waterfall,
702
00:48:37,351 --> 00:48:41,284
rapidly eroding
the waterlogged chalk.
703
00:48:44,084 --> 00:48:46,984
We'd have had initially
a small stream of water
704
00:48:47,018 --> 00:48:49,818
coming over
the top of the rock ridge,
705
00:48:49,851 --> 00:48:51,451
that would have
catastrophically crumbled,
706
00:48:51,484 --> 00:48:54,184
with large amounts of rock
being removed
707
00:48:54,218 --> 00:48:57,751
and more and more water
flooding through,
708
00:48:57,784 --> 00:49:00,084
just running away with itself.
709
00:49:00,118 --> 00:49:03,418
From the shape of the features
on the sonar,
710
00:49:03,451 --> 00:49:07,984
Jenny estimates that
the floodwaters raced through
711
00:49:08,018 --> 00:49:13,651
at a rate of about
264 million gallons a second.
712
00:49:16,851 --> 00:49:21,918
That's almost 60 times the flow
rate of the Mississippi River.
713
00:49:26,618 --> 00:49:28,784
You would have seen
a tidal wave overtopping
714
00:49:28,818 --> 00:49:32,151
and washing a giant gorge
into that landscape.
715
00:49:32,184 --> 00:49:35,618
The deluge crashed on,
716
00:49:35,651 --> 00:49:37,518
breaking through the chalk ridge
717
00:49:37,551 --> 00:49:39,884
linking today's
Britain and France,
718
00:49:39,918 --> 00:49:44,051
before finally reaching
the Atlantic Ocean.
719
00:49:44,084 --> 00:49:48,651
It was this cataclysmic flow
that created the English Channel
720
00:49:48,684 --> 00:49:51,251
and began the process of erosion
721
00:49:51,284 --> 00:49:54,484
that led to what's now Britain
722
00:49:54,518 --> 00:49:58,418
becoming an island
for the first time.
723
00:50:02,251 --> 00:50:07,884
The clues in Iceland,
the English Channel,
724
00:50:07,918 --> 00:50:13,151
and the Channeled Scablands
of Washington state
725
00:50:13,184 --> 00:50:15,418
reveal that floods bigger
and more devastating
726
00:50:15,451 --> 00:50:18,584
than anything we see today
727
00:50:18,618 --> 00:50:23,818
have torn across and
helped shape Earth's surface.
728
00:50:23,851 --> 00:50:28,618
These giant mega-floods
totally shaped a landscape
729
00:50:28,651 --> 00:50:30,618
in a matter of days or weeks.
730
00:50:32,384 --> 00:50:33,951
But the question is:
731
00:50:33,984 --> 00:50:38,618
could a flood on this scale
happen again?
732
00:50:41,551 --> 00:50:44,251
The one thing all these
mega-floods have in common
733
00:50:44,284 --> 00:50:47,918
is that they involve
huge volumes of ice melting
734
00:50:47,951 --> 00:50:52,218
and being released
in one sudden burst.
735
00:50:56,018 --> 00:51:00,818
In Iceland,
a volcano beneath the ice sheet
736
00:51:00,851 --> 00:51:04,551
could trigger a mega-flood
at any moment.
737
00:51:09,584 --> 00:51:12,018
Fortunately, very few people
738
00:51:12,051 --> 00:51:14,751
live in the Icelandic
flood zone,
739
00:51:14,784 --> 00:51:19,251
and the huge volume of ice
needed to create glacial lakes,
740
00:51:19,284 --> 00:51:21,651
on the scale of the ones
that carved the English Channel
741
00:51:21,684 --> 00:51:22,651
and the Scablands,
742
00:51:22,684 --> 00:51:28,584
can only build up
during ice ages.
743
00:51:34,051 --> 00:51:36,284
But there is one region
on Earth today
744
00:51:36,318 --> 00:51:39,518
where stores of melting ice
745
00:51:39,551 --> 00:51:43,184
still pose a major flood risk
to millions:
746
00:51:43,218 --> 00:51:46,784
ice- and snow-covered mountains.
747
00:51:49,218 --> 00:51:50,784
Wherever you have glaciers,
748
00:51:50,818 --> 00:51:52,151
you have a lot of water.
749
00:51:52,184 --> 00:51:55,551
Wherever you have glaciers
in a mountain,
750
00:51:55,584 --> 00:52:00,551
you have the high likelihood of
making a glacially dammed lake,
751
00:52:00,584 --> 00:52:03,418
and those glacially dammed lakes
are unstable
752
00:52:03,451 --> 00:52:05,284
and could drain
catastrophically.
753
00:52:07,584 --> 00:52:09,651
We're not going to get, today,
754
00:52:09,684 --> 00:52:12,118
releases of water
like Lake Missoula,
755
00:52:12,151 --> 00:52:14,018
that was 2,000 feet deep.
756
00:52:14,051 --> 00:52:19,318
But we can get glacial lakes
that are a hundred feet deep,
757
00:52:19,351 --> 00:52:20,584
and these will produce
758
00:52:20,618 --> 00:52:23,618
really dangerous
and spectacular floods.
759
00:52:26,051 --> 00:52:31,218
Today, floods in populated areas
wreak untold devastation.
760
00:52:31,251 --> 00:52:35,051
Faced with Hurricane Harvey's
impact on Houston
761
00:52:35,084 --> 00:52:37,451
and the floods in Bangladesh,
762
00:52:37,484 --> 00:52:40,684
it may seem that floods
could not get any worse.
763
00:52:40,718 --> 00:52:44,951
But the vast floods of the past
carved huge features
764
00:52:44,984 --> 00:52:48,851
into the very bedrock
of continents.
765
00:52:48,884 --> 00:52:50,918
Those scars are a stark reminder
766
00:52:50,951 --> 00:52:55,351
of just how destructive
floods can be.
767
00:53:02,718 --> 00:53:03,717
Look at that guy.
768
00:53:03,718 --> 00:53:05,117
Armed and dangerous.
769
00:53:05,118 --> 00:53:06,850
There's an animal
with an incredible weapon!
770
00:53:06,851 --> 00:53:08,417
And fighting for their lives.
771
00:53:08,418 --> 00:53:10,517
Weapons that are used
in fighting
772
00:53:10,518 --> 00:53:11,817
and weapons that are big.
773
00:53:11,818 --> 00:53:13,483
How did they evolve this way?
774
00:53:13,484 --> 00:53:16,617
And why do only some creatures
have them?
775
00:53:16,618 --> 00:53:18,650
Some have these huge,
spectacular weapons
776
00:53:18,651 --> 00:53:20,383
and others have nothing at all.
777
00:53:20,384 --> 00:53:22,117
An arms race in the wild.
778
00:53:22,118 --> 00:53:23,850
- Wow, look at that!
- Out of nowhere.
779
00:53:23,851 --> 00:53:25,484
"Extreme Animal Weapons."
780
00:53:27,084 --> 00:53:31,718
Coming soon on "NOVA."
781
00:53:37,318 --> 00:53:39,851
This "NOVA" program
is available on DVD.
782
00:53:39,884 --> 00:53:45,384
To order, visit shopPBS.org
or call 1-800-PLAY-PBS.
783
00:53:45,418 --> 00:53:48,418
"NOVA" is also available
for download on iTunes.
62050
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