All language subtitles for Drain the Oceans Series 1 Deadly Pacific 720p

NARRATOR:
Deep below the Pacific Ocean

lies a tectonic monster.

MAN: You know how we told you
not to panic?

Well, that's all off.

Panic. Run for your life.

(screaming)

NARRATOR: The world's biggest
tsunami machine,

its dangers hidden from view,

until now.

Imagine if we could
empty the oceans,

letting the water drain away

to reveal the secrets
of the seafloor.

Now, we can,

using the latest underwater
scanning technology,

piercing the deep oceans,

and turning accurate data
into 3D images.

This time, is the world's
next mega-earthquake

brewing
off the coast of Seattle?

MAN: We can have the big one

right now, as we speak.

NARRATOR:
Why has the discovery

of an underwater volcano,
just 44 miles from Tokyo,

got seismic experts so worried?

And what do scientists caught
up in a undersea earthquake

learn about the dangers
facing New Zealand?

MAN: The front of it would have
a huge amount of energy.

The scale is just immense.

NARRATOR: Drain the Oceans

reveals the secrets
of the deadly Pacific.

(music)

(music)

The Pacific Ocean--

the planet's largest
and deepest.

500 million people
live around its coast.

But deep beneath the waves
lurks a terrifying monster

that threatens all of them,

the planet's largest and
most dangerous seismic zone--

the Ring of Fire.

90 percent of all earthquakes
strike here

and when they do,

the consequences
are devastating.

(siren)

(siren)

March 11, 2011.

(speaking Japanese)

NARRATOR:
Tohoku, northeastern Japan.

(shouting)

A magnitude 9 earthquake
violently shakes the region.

MAN: Oh!

NARRATOR: The quake causes
buildings to collapse

and people to flee
for their lives.

(shouting)

But that's not the end
of the destruction.

A defining characteristic
of the Ring of Fire

is that
the biggest earthquakes here

shake not just the land,
but the seabed as well...

...generating
huge surges of water...

(speaking Japanese)

NARRATOR: ...deadly tsunamis.

(shouting)

(music)

(music)

(shouting)

(shouting)

(music)

(music)

In Japan, almost 16,000 people
are killed.

And geologists are certain
it will happen again

somewhere around
the Pacific Ring of Fire,

but when and where?

Pulling the plug
on the Pacific Ocean

reveals a glimpse of one
of its most terrifying secrets,

just off the west coast
of America--

not the infamous
San Andreas Fault

in California,

but something close
to the teeming cities

of Portland,
Seattle, and Vancouver,

something far more menacing.

(music)

LELAND O'DRISCOLL: It's known
that it's an active structure.

It could happen now, and that's

truly the source of fear.

(music)

NARRATOR:
This region of North America

is known as Cascadia,

one of the most beautiful
and peaceful in North America.

It's also home
to 10 million people.

They feel safe here,
and why wouldn't they?

In all recorded history,
there's been no trace

of any natural disasters
in the region,

but on the 18th of May 1980,

everything changes.

(rumbling)

MAN: There is no question at all

that the volcanic activity
has begun.

NARRATOR: Mount St. Helens,
50 miles from Portland.

Huge eruptions blast
a 1,300-foot crater

in the summit of a volcano
long thought to be dormant.

REPORTER: It is the biggest
eruption ever recorded

of Mount St. Helens so far.

REPORTER:
I think it does not look good

for people in that area now.

NARRATOR: It's the worst
volcano disaster

in U.S. history,
claiming 57 lives.

(music)

(music)

Scientists are now forced
to ask a tough question.

MAN: Is it, is it scaled
to look at tsunamis?

NARRATOR: Does the eruption

of the supposedly inactive
Mount St. Helens

mean the 10 million inhabitants
of Cascadia

are now sitting
on a seismic time bomb

that just started ticking?

To find out, scientists
launch an urgent investigation

on land
and deep under the sea.

Off the coast of Cascadia,
the latest scanning technology

reveals something extraordinary
and terrifying.

Based upon their exact data,
for the first time,

we can pull the plug
on the Pacific Ocean

to reveal
their shocking discovery.

As the water drains away,

a vast world no human has
ever seen before is laid bare.

For mile after mile
across its heart,

the Pacific Ocean floor

is a vast, flat,
featureless plain...

...until suddenly,

the landscape changes.

Here, just 150 miles

from the great cities
of the Pacific Northwest,

a dramatic transformation
takes place,

revealing a fractured,
broken seafloor...

...brooding,

menacing.

O'DRISCOLL: One of the best
analogies I can think of

is looking
at the, the front end of a car

that's been
in a head-on collision,

dented and corroded
and corrugated

into this complex mess.

NARRATOR: The scale of
this geological fender bender

is truly epic.

Colossal cliffs,

almost twice the height
of the Grand Canyon...

...some deep enough to hide
six Empire State Buildings

stacked on top of each other,

all of it stretching 700 miles
along the Cascadian coastline.

This titanic rupture
in the earth's crust

is what geologists call
a subduction zone.

At the core of our planet

is a boiling cauldron
of molten iron,

surrounded
by a thick mantle of rock.

That outer crust is divided
into huge tectonic plates

that are constantly
in motion.

When they collide,
they thrust mountains upwards

and carve deep trenches
into the ocean floor

as one plate drives the other
back inside the earth.

If the plates lock together,

tremendous pressure
begins to build.

O'DRISCOLL:
Only in subduction zones

where the plates
are compressing together

and there's this lock
between the plates,

that, those are
the only fault zones

that have the potential

to create magnitude

9.0 and higher events.

NARRATOR: But that's not all.

Subduction zones,
because they are underwater,

can generate not just
earthquakes, but huge tsunamis.

Written history suggests
that Cascadia's subduction zone

has been inactive
for centuries,

but does written history
go back far enough?

Is this monster really
as sleepy as it seems?

(music)

Scientists scour
the landscape around Cascadia

in search of answers,

and, in the banks
of coastal rivers like this,

they find an alarming pattern.

DAVID YAMAGUCHI: It's very much
like a chocolate cake.

You slice a chocolate cake,

and there's a layer
of brown frosting,

and then, down below, there's
another layer of brown frosting,

and the Washington coast
looks very much like that.

NARRATOR:
But there's nothing sweet

about these layers
of peat and sand.

YAMAGUCHI:
Each of these buried soil layers

on the Washington coast

reflected times when the coast
had been jerked downwards

by the occurrence
of an earthquake.

NARRATOR:
In the layers of peat,

David Yamaguchi
and geologist Brian Atwater

unearth evidence
of seismic devastation

stretching back
over thousands of years,

quakes that scarred the entire
coastline of Cascadia.

The question now is,

when was the last one,
and how big was it?

NARRATOR: The Copalis River,
Washington State.

Scientist David Yamaguchi
is on a quest.

He and his fellow scientists
already suspect

that the 10 million inhabitants
of Cascadia

are living
on a seismic time bomb,

but when did it last go off,

and how big was it
when it did?

Three miles inland, he comes
across a haunting sight.

(music)

(music)

This is what's left
of a once mighty forest

of red cedar
and spruce trees,

thousands of petrified corpses

scattered
over hundreds of miles

along Cascadia's coast.

YAMAGUCHI: And the mystery here

is what killed the trees,

and when did they die?

(chainsaw)

NARRATOR:
Sampling dozens of the trees,

Yamaguchi analyzes
their age rings,

and what he finds
is a new mystery.

YAMAGUCHI: The trees were fine
and healthy in 1699.

Then, the following summer,

the trees that, that would
normally have produced

another annual ring

were nearly all dead.

NARRATOR: What happened
to kill an entire forest

in one instant?

Another scientist
thinks the answer could lie

deep in the geological
car crash

of the Cascadia
subduction zone,

just 100 miles away.

(music)

Chris Goldfinger is an
underwater earthquake detective

seeking evidence
in core samples

taken from deep
under the seafloor.

Studying the layers of sediment

reveals when and where
earthquakes struck in the past

and how powerful they were.

CHRIS GOLDFINGER: Sediment
settles to the seafloor,

but it got there somehow,

from some storm, an earthquake,
and even a volcanic eruption

will leave a deposit
on the seafloor,

and so it's just
a very precise,

three-dimensional
tape recorder,

just recording
every single thing that happens.

Well, this is a core sample

from, from the seafloor
off of Cascadia,

and it's about
2,000 years' worth of sediment.

NARRATOR:
In the middle of the sample,

dating to roughly
300 years ago,

Goldfinger finds the classic
signs of a large earthquake.

GOLDFINGER: Well, in fact,
the shells like this

turn out to be
shallow-water fossils,

but this is a deep-water core,

so these things
were carried down

from shallow water
to deep water.

What this points to is
a fairly dramatic kind of event.

(rumbling)

NARRATOR: That dramatic event
was an earthquake

that triggered
a vast underwater landslide.

And it all happened
around the same time

that Yamaguchi's trees died.

Next, he compares core samples
of the same date,

taken along the 700 miles

of the Cascadia
subduction zone.

Core after core reveals
the same disturbed layer,

all occurring
at the exact same time.

This is evidence
of no ordinary earthquake,

but a monster.

GOLDFINGER: Cascadia, through
the study of past earthquakes,

has shown that it's capable
of generating earthquakes

that run the full length,
roughly 1,000 kilometers.

NARRATOR: In 1700,
the Cascadia subduction zone

generated a cataclysmic quake
along its entire length,

with a magnitude that
Goldfinger and other scientists

calculate as level 8.

But that's not all.

This was more
than just an earthquake.

Japan has a long history
of earthquakes and tsunamis,

with written evidence
going back 1,400 years.

Among these historical records,

in accounts written
by samurai and merchants,

researchers uncover reports
of a 16-foot tsunami

that arrives
from far out in the Pacific,

swamping the eastern coast.

It destroys houses

and causes villagers
to flee to higher ground.

The date of these reports?

1700.

YAMAGUCHI: They propose that
our earthquake was magnitude 9.

A magnitude 8
was not big enough

to produce a wave
that's that wide.

In addition,

based on the arrival times
of the waves in Japan,

the earthquake
must have happened here

on January 26, 1700

at 9:00 at night,

yes, local time.

NARRATOR: The Cascadia
mega-quake was so powerful

that it generated a tsunami

that hit Japan,
4,500 miles away.

Using the combined insights

of experts from both sides
of the Pacific

and the extraordinary
drained landscape

of the subduction zone,

it's now possible to visualize

the titanic events
of January 1700

and its deadly trail
of destruction.

(music)

GOLDFINGER:
The entire subduction zone

shakes very hard.

The ground motion is big,
and it's over a big area,

so it's gonna shake
a lot of sediment loose.

(music)

There are gonna be
hundreds of landslides.

They're gonna all
come tumbling down the slope

and leave a deposit.

(music)

(music)

NARRATOR: The quake
also triggers a huge tsunami

which smashes
into the coast of Cascadia,

flooding the land
with seawater,

including the red cedar forest

and everything else
in its path.

Although
there is no written history

of the 1700 earthquake
and tsunami in Cascadia,

native peoples of this region

do hold haunting memories
of these terrible events.

GOLDFINGER: The First Nation
peoples had oral histories

about the great battle
of Thunderbird and Whale

that had a lot of shaking
of the ground,

washing away of villages.

MAN: The Great Thunderbird

finally carried the Whale
to its nest.

There were a shaking,
jumping up,

and trembling
of the earth beneath

and the rolling up
of the great waters.

GOLDFINGER: So, once
these stories came to light,

the Native Americans
essentially said,

'Why didn't you just ask us?
We knew all about that.'

The story had a moral,
and the moral was,

don't build your village
too close to the beach.

NARRATOR: But evidently,
no one was listening.

Today, 10 million people live
along the coast of Cascadia

and in the cities of Seattle,
Portland, and Vancouver,

and the question is
how likely is it

that the monster
will roar again?

THOMAS HEATON: It's not at all
like a clock,

but the average time

between the earthquakes

is on the order of 400 years.

NARRATOR:
If the Cascadia subduction zone

repeats its 400-year cycle,

then its next mega-quake
could be imminent.

O'DRISCOLL:
We know the strain is building,

and we know, one day,
it's gonna be released,

and that could happen any day.

NARRATOR: So, what will happen
when the monster wakes?

GOLDFINGER:
In a subduction zone earthquake,

energy is stretched out,
not 10 seconds or 20 seconds,

more like three minutes.

HEATON: The shaking's not

incredibly violent,

but it lasts for a long time,

and it's a very long,

rolling motion.

For tall buildings,
that's not good news.

NARRATOR:
With buildings collapsing

and infrastructures shattered,

it will be hard
for the population to escape

from the coastal area,

the worst place to be trapped.

HEATON: You know that
if you get a giant earthquake,

you'd better assume you're

about to get a giant tsunami.

NARRATOR: The tectonic plates'
sudden release of pressure

forces a massive swell
in the ocean above.

GOLDFINGER:
So, the wave height, initially,

might only be
a couple of meters,

but it might be
200 kilometers long,

and it heads out
at the deep-water wave speed,

which is very fast,
so it's jetliner speed,

500 or 600 miles an hour.

Everything we've seen in Japan,
we can expect, basically,

the exact same thing
in Cascadia.

(man on loudspeaker)

HEATON:
30 meters of water rushing in

at 10 meters a second
kind of velocities...

...it just scours

everything off the,

off the surface of the earth.

NARRATOR: Within 20 minutes,

the tsunami will approach
the coast of Cascadia.

GOLDFINGER: When that wave
starts feeling bottom,

like all other waves do,

it gets compressed
from 200 kilometers

down to a kilometer or two.

HEATON: It's like an enormous

tide happening very quickly.

(music)

NARRATOR: A Cascadia mega-quake
is predicted to be

North America's
worst ever natural disaster.

O'DRISCOLL: The fatalities
could be fairly large.

We're looking at, at a few tens
of thousands of people,

at least, on our coastline.

We know the strain is building,

and that's quite harrowing
to know

that we could have the big one
right now, as we speak.

NARRATOR: But Cascadia is not
the only populated region

in the Ring of Fire
danger zone.

As the waters of the Pacific
continue to drain away,

they reveal
another sleeping giant

near the east coast
of New Zealand,

where scientists discover
traces of an epic seismic event

that reshaped
the Southern Hemisphere

and ask,
when will it happen again?

NARRATOR:
The islands of New Zealand

lie directly
on the Ring of Fire.

Small earthquakes shake
the ground here every week.

(sirens)

(screaming)

In 2011, the country's second
largest city, Christchurch,

is devastated
by a magnitude 6.3 quake...

WOMAN: I can confirm
that there have been deaths.

NARRATOR:
...killing 185 people.

(siren)

100 miles from Christchurch

is the town of Kaikoura.

MAN: Kaikoura, welcome aboard.

WOMAN: Thank you.

NARRATOR:
A peaceful holiday resort

that's popular
with nature lovers.

MAN: Keep looking out there.

You'll see them just
out of the front of us there.

NARRATOR:
Its deep, nutrient-rich waters

entice whales
close to the shore.

MAN: This is
the beautiful humpback,

and it is awesome to see humps.

NARRATOR:
While these leviathans

are Kaikoura's
star attraction,

geologists now come here
for a very different reason.

New Zealand's geologists
constantly monitor

the threat posed
by the faults and fissures

that run under the islands,

trying to discover where the
next big quake might strike.

Ground surveys can help,

but investigating
under the waves

has always been a challenge,

until now,

because the latest
multiple-beam sonar technology

is opening up a hidden world.

JOSHU MOUNTJOY: It's multiple
beams of sound energy

being released from
the instrument on the vessel,

and we time how long it takes
for that beam to come back.

(beeping)

We do what we call
mowing the grass,

and we're just collecting
this continuous image

of what the shape
of the seabed is.

NARRATOR: And the shape
of the seabed near Kaikoura

is not what they expected.

As the data comes in,
it slowly reveals

something astonishing
and potentially lethal.

Pulling the plug
on the Pacific Ocean

exposes the remarkable sight.

Just a half mile
from the town of Kaikoura,

as the water drains away,

it reveals the plunging walls
of a vast, undersea chasm...

...the Kaikoura Canyon.

MOUNTJOY: The Kaikoura Canyon,

it comes within
a really short distance

of the coastline out here,
less than a kilometer,

and, from there,
it drops down at 30, 40 degrees

down to about 600 meters
water depth.

Then it carries on down to
about 2,000 meters water depth.

NARRATOR:
Stretching for over 37 miles,

the canyon forms part

of one of the deepest
sea channels in the world.

The walls nose-dive
over 6,000 feet--

the depth of the Grand Canyon.

Geologists wonder at
the immense geological forces

involved to create
such a huge canyon...

...but also ask themselves,

does it pose a threat to those
living on the nearby coast?

Geologists know that landslides

triggered by earthquakes
in underwater canyons

can generate lethal tsunamis.

Could the Kaikoura Canyon
do the same?

On board the research ship
Tangaroa, they investigate.

MAN: Let's try it again, Mitch.

(radio chatter)

NARRATOR: Using Perspex tubes,
they gather evidence

from the sedimentary layers
on the canyon's floor,

looking for signs of seismic
activity in the past.

MAN: Yep.

MAN: This is core two.

NARRATOR: But then, just
as they're studying history,

they get the chance
to make some.

Midnight, November 14, 2016.

(static)

WOMAN: And we are undergoing
a fairly dense earthquake

at the moment,

so, please,
just get to somewhere

where you are safely
under some protection.

I can honestly say,
I doubt that I'll be able

to stay in the chair
for much longer.

(alarm)

NARRATOR: The 7.8 magnitude
quake rips apart the earth,

creating giant crevices and
billions of dollars of damage.

Geologist Joshu Mountjoy has
never seen anything like it.

MOUNTJOY: The key thing was
that it just covered

such a large geographic area,

and it was right
on our coastline.

NARRATOR: The quake rattles
the ground for two minutes,

causing thousands
of landslides

across 3,800 square miles.

Just north of Kaikoura
is the biggest.

800 million cubic feet
has shaken to the ground,

exposing a virgin cliff face
almost a mile wide.

MOUNTJOY: Grass and tussock
we're standing on

was connected to
that grass and tussock up there,

and then the whole block
slid right down

to bring it down
to where it is now,

so this has happened
during the earthquake.

We call that co-seismic,
and there's actually

a fault line
running through here

that runs
right down to the coastline.

NARRATOR:
The shaking is so severe

that it thrusts the foreshore
of Kaikoura

upwards by three feet
in just one second.

MOUNTJOY: We're looking at here,

we can see
a whole a lot of seaweed

that's stranded
above the tide line.

Before the earthquake,

that would have been
down below the sea level.

NARRATOR: But Joshu Mountjoy
isn't only interested

in how the earthquake
tears apart the land.

As soon
as the earthquake begins,

he wants to know
the effect it's having

deep under the ocean,

in the newly scanned
Kaikoura Canyon.

Mountjoy makes contact with
the research ship Tangaroa,

which is mapping
the very far end of the canyon.

MOUNTJOY: We requested
that they divert their energy

to focus on doing
some earthquake response.

NARRATOR: That response is to
immediately gather core samples

from the floor of the canyon
before the dust has settled.

ALAN ORPIN:
So, we really had

an unprecedented opportunity
to assist.

Well, what, what is the
aftermath of a major earthquake?

NARRATOR:
If the crew responds in time,

they can achieve a world first,

recording the impact
of an earthquake

in a deep-sea canyon
as it happens.

NARRATOR: Within minutes
of the earthquake,

the crew retrieves precious
core samples from the seafloor.

MOUNTJOY: They collected
the sedimentary deposit

that had only just happened,
so they were boring it

while it was probably still
settling out of the ocean.

NARRATOR: And what marine
geologist Alan Orpin discovers

is intriguing.

ORPIN: What we found was
a very soupy, fluidized mud

that was lying
on the seabed.

As we moved south,

what we found
was compelling evidence

of core after core
for a thickness of material

that had spilled down
through the channel.

NARRATOR:
The more samples they take,

the clearer
the picture becomes.

The shaking of the quake
has created a secondary

and truly spectacular
undersea upheaval.

MOUNTJOY: No one's ever
been able to observe one

of these huge landscape
scale canyon-flushing events

to show exactly
what effect it had

on the canyon itself.

(music)

NARRATOR: Using the data

gathered by the crew
of Tangaroa

and the remarkable
drained seascape,

the impact
of a large earthquake

on a gigantic underwater canyon

can now be visualized
for the very first time.

The seabed shudders ominously
as huge tectonic forces

power through
the earth's crust.

(rumbling)

This dislodges
a top layer of sediment,

becoming a gigantic
canyon-flushing landslide.

(rumbling)

(music)

Rock and sediment
tumble down the steep walls.

850 million tons of debris
roars through the canyon,

gouging 160 feet
out of the floor.

MOUNTJOY: There's really like
a powder snow avalanche.

The front of it would have
a huge amount of energy

moving at about
70 kilometers an hour

and maybe 400 meters high.

It's spreading
tens of kilometers

across the ocean floor.

It's just gonna take over
everything that's down there.

NARRATOR: The landslide travels
with such velocity,

it carves a path
across the seafloor

for an astonishing 435 miles.

A landslide of this ferocity
has the potential

to generate a tsunami
that could devastate Kaikoura.

MOUNTJOY: When a landslide
creates a tsunami,

you need a large chunk
of material to slide downslope,

and what happens is it's
coupled to the water above it

and actually
pulls it down with it,

and then,
as that water rebounds,

then you create your wave.

NARRATOR:
This time, Kaikoura is lucky.

No large objects
fall off the canyon rim.

Next time, the town may not
be so fortunate.

And the dramatic events of 2016
draw the geologists' attention

to an even bigger
tsunami threat nearby.

By continuing
to drain the ocean

off the northeast coast
of New Zealand,

we can see the reason
for their fear.

Lurking just north
of the canyon

and frighteningly close

to the New Zealand capital
of Wellington--

the Hikurangi subduction zone,

2,174 miles
of seismic trouble.

Geologists recently learned
that stress has been building

at its southern end
for hundreds of years,

and they believe
that the 2016 earthquake

has ratcheted the pressure
up even further.

Understanding
what's happening here

has become
an international priority.

100 ocean floor
seismic measuring devices

and support scientists
recently arrived from Japan.

They're here to monitor and
record the mounting stresses

in the hope of discovering
the true danger of Hikurangi.

(music)

Geologists delve deep

into the subduction zone's
tectonic past.

And their results
are astounding.

It hasn't fractured recently,
but deep in its past,

it produced something bigger
than anyone had imagined.

Now, using
the geologists' data,

it's possible to reconstruct,
for the first time,

a tectonic event
that reshaped a hemisphere--

the Ruatoria avalanche.

Just 200 miles from
what today is New Zealand's

bustling capital, Wellington,

the epicenter of one
of the biggest seismic events

in the history of the earth.

(rumbling)

170,000 years ago,
the Australian plate

is bearing down
on its Pacific neighbor,

grinding down
everything in its path.

Even a towering
volcanic cone

six times the volume
of Mount Fuji

becomes collateral damage.

The resulting seismic shock

triggers
an underwater landslide

on a scale rarely seen.

Moving a swath of broken rock,

the equivalent
of three-quarters the size

of the Grand Canyon.

(rumbling)

(rumbling)

MOUNTJOY:
So, the Ruatoria avalanche

is the second biggest submarine
landslide in the world.

It's absolutely enormous,
3,000 cubic kilometers.

And if you can try and visualize
a cubic kilometer,

that's a kilometer high

by a kilometer long
by a kilometer wide,

3,000 of them.

(music)

NARRATOR: Nobody knows when
the Hikurangi subduction zone

will facture again,

but when it does,

geologists estimate the tsunami
wave from the mega-quake

will take only seven minutes

to reach New Zealand's
capital city, Wellington,

and its
half a million residents.

And such a huge tsunami

wouldn't only impact
New Zealand.

There would be
an ocean-wide alert

as the waves fan out
into the Pacific.

(screaming)

The last time a tsunami
on this scale

hit the southern arc
of the Ring of Fire

was December 26, 2004,

devastating 14 countries

and killing 230,000 people--

a terrible reminder of the
power of the deadly Pacific.

Continuing to drain away
the Pacific Ocean

reveals a very different kind
of seismic threat

5,000 miles away,

right next to one of the most
densely populated cities

on the planet.

Just 44 miles
from bustling Tokyo,

a mysterious underwater feature

has scientists
on the edge of their seats.

The island nation of Japan

suffers from its position
on the Ring of Fire.

The country accounts
for about 20 percent

of the world's earthquakes
of magnitude 6 or greater.

Its biggest city, Tokyo,

has been hit
by nine mega-earthquakes

in the past 95 years.

But just 44 miles
off the city's shoreline,

geologists
have recently discovered

a new and very different
tectonic troublemaker.

The first discovery is made
by scientist Ken Tani.

He's heading out to test

his new underwater remotely
operated vehicle, or ROV,

in the waters
just off the island of Oshima,

south of Tokyo Bay.

Until now, the waters here have
been considered safe passage

for one of the busiest
shipping lanes

into the port of Tokyo,

with 400 vessels
passing through here every day,

but that is about to change.

NARRATOR:
44 miles from Tokyo Bay,

Ken Tani prepares an underwater
survey, using an ROV.

(speaking Japanese)

Passing over a rocky mound
on the seafloor,

he discovers
something extraordinary,

a seismic beast
that has lain hidden

for hundreds of thousands
of years.

Using Ken Tani's own data,
draining away the Pacific Ocean

reveals what is lurking
below the waves.

As the water recedes,

it exposes
a large flat plateau.

Near its center,

what looks like an ordinary
hole in the seafloor.

But on closer inspection,
the frightening truth emerges.

It's the vent
of an undersea volcano.

Thousands of years ago,

it very likely
had a towering cone,

but erosion and wave action
has worn it down,

which is why
no one discovered it before.

But is it as dormant
as it appears?

Ken wastes no time
in exploring the crater.

KEN TANI:
So, that's the sea surface.

NARRATOR: Testing the water
temperature around the outlet

with a submarine thermometer,

and what he finds
surprises everyone.

(music)

NARRATOR:
The smoldering undersea volcano

is named Oomurodashi,

44 miles south of Tokyo,

placing at risk the lives
of thousands of mariners

in the shipping lanes above.

In 1952, an underwater volcano
like Oomurodashi

erupts 260 miles
south of Tokyo.

Super-heated magma
reacts with water

to produce what's called
a phreatomagmatic explosion,

an underwater steam bomb.

A nearby ship is engulfed
in the explosion,

killing all 31 on board.

NARRATOR:
A stirring volcanic giant

is not just
a danger to shipping.

A major eruption
could send tsunami waves

racing towards Tokyo,
population 38 million.

NARRATOR: Ken decides he needs

to keep Oomurodashi
under surveillance.

Nothing happens for five years.

Then, when he returns
to gather fresh samples,

Oomurodashi has another
unpleasant surprise for him.

Returning
to the drained ocean floor

reveals his next
shocking discovery.

In addition to the crater,

five more volcanic cones.

That's five more possible
outlets for the volcano.

Only one is currently active,
but all could become so.

And if the volcano
keeps growing,

Oomurodashi could become
an even greater threat

by breaking the surface

and spewing toxic ash
into the sky.

Active volcanos can do this
astonishingly quickly.

It took Nishinoshima,
Japan's latest volcanic island,

just two years to grow

from a flat-topped,
underwater summit

to this tectonic monster,

measuring one square mile
and 443 feet high.

But Nishinoshima

is nearly 600 miles from Tokyo.

Oomurodashi is only 44.

Once out of the water,

any major eruption
of Oomurodashi

could shower Tokyo
in choking volcanic ash.

Despite the dangers,
Ken Tani and his team

continue
to monitor Oomurodashi.

Any increase
in the volcano's activity

could signal
impending disaster

for the thousands living
on nearby islands

and threaten the safety
of millions in nearby Tokyo.

Draining the Ring of Fire

has revealed
hidden geological wonders

and terrors,

exposing new evidence
of this immense power

of plate tectonics

and its ability to create

and destroy.