All language subtitles for How the Earth Was Made HC 720p HDTV S01E09 Tsunami EN

Earth, a 4.5- Billion-year-old planet,
still evolving.

As continents shift and clash, volcanoes
erupt, glaciers grow and recede,

the Earth's crust is carved
in countless fascinating ways,

leaving a trail of geological
mysteries behind.

Water.

One of the most powerful
forces on the planet.

It plays a crucial role in creating life
and destroying it,

in forging landscapes
and in breaking apart the Earth.

In its most dramatic form, it becomes
a killer wave known as a tsunami.

Until recently, predicting when
these monsters may next strike

has been impossible.

But today, scientists are starting
to understand these giant waves.

By connecting clues as varied as ancient
Japanese writings and landslides,

ancient corals and buried
Native American settlements,

the secrets of tsunamis
are finally being unlocked.

Tsunamis. One of the most
deadly forces of nature.

Giant waves that travel faster
than a jet plane,

they can cross entire oceans
in just hours.

They have the power to smash buildings,
vehicles, anything in their way.

MAN: By itself, you wouldn't
think that water just streaming

into the coast would necessarily
cause so much damage.

But in fact, they are very fast moving
and they pick up everything in its path,

so it's not the water by itself,
it's what comes with the water

that is also a part of the big hazard.

A tsunami isn't over
in just a few seconds,

it is a torrent of raging water
that keeps coming.

The main thing about a tsunami
is the persistence.

It comes on and on and on,
and just when you think it has to quit,

it keeps coming, and it's the
power plus the... the duration

that is unstoppable, really.

Tsunamis have ravaged
the Earth for billions of years.

When the Earth was first created,
the moon was much closer.

It filled the sky.

Its gravitational pull
was much stronger,

and it generated towering waves
over half a mile high

that raced across the primeval oceans.

MAN: Oh, my God!

(SCREAMING)

Today, tsunamis are still a threat
to coastlines all over the world.

MOONEY: Tsunamis will always occur,
and have always occurred,

throughout Earth's history.

But it's only been more recently,
as population densities have increased

and people have moved
and migrated to the coastal regions,

that we've become much
more aware of the tsunami hazards.

The investigation into what
caused these monster waves

began over a thousand years ago

on the islands of Japan.

This country is the world's
tsunami hotspot.

Its coasts have been pounded
with these enormous waves

more than anywhere else on the planet.

Evidence for this
is the word tsunami itself.

It is Japanese and literally
means "harbour wave".

Japan has the longest written tsunami
record of anywhere in the world.

The records go back as far as 684 A.D.

By studying these records,
it is possible to work out that,

on average, this country has been
struck nearly every seven years.

Samurai writings speak of people
living on the coasts

running for higher ground
as soon as they felt an earthquake.

The Japanese knew this was a clue,

a warning sign that a deadly
tsunami would soon follow.

But despite their attempts to escape,

tsunamis have continually brought death
and destruction to these islands.

In 1896, a wave that hit Honshu
in the northeast

claimed the lives of 27,000 people.

In 1933, the same area
was smashed again.

This time, 3,000 people
were swept away.

And in 1993, the island of Okushiri
was rocked by an enormous earthquake

measuring 7.8 on the Richter scale.

Buildings were levelled and fires raged.
But worse was to come.

Minutes after the shaking had subsided,

an ominous white crest
appeared on the horizon - a tsunami.

A gigantic wave swept in,
flattening any buildings still standing.

In Japan, the locals had already
worked out the connections

between earthquakes and tsunamis.

But there's another hotspot on Earth
where tsunamis regularly strike -

the Hawaiian islands.

But very few of them were preceded
by an earthquake.

The city of Hilo on the Big Island
has been dubbed

the Tsunami Capital of the World.

Dozens of these enormous waves
have hit these beautiful islands,

and the mystery is why.

With no natural warning to go on,

the people of Hawaii must rely

on the world's biggest
tsunami monitoring station.

Set up in 1949, it is connected
to a network of buoys

spread across the Pacific Ocean.

These buoys provide important clues.

They monitor changes in sea level

that indicate the approach
of any potential tsunamis.

In 1960, scientists got the breakthrough
they were looking for.

They were finally able to work out
the type of event

at the root of Hawaii's
mystery tsunamis.

An enormous quake on the coast of
Chile, the biggest recorded of all time,

with a factor 9.5 on the Richter scale,

triggered a tsunami that swept
across the entire Pacific Ocean

in just a few hours.

The islands of Hawaii were thousands
of miles away, directly in its path.

The Tsunami Warning Center
was monitoring its progress,

revealing for the first time
that a single massive wave

crossed thousands of miles of ocean.

The warning centre was a success.

They were able to evacuate
the communities closest to the shore

before the wave struck.

But the homes they left
behind were decimated.

In Hilo, the tsunami was so strong
it even bent parking metres in half.

The wave continued
past Hawaii to Japan.

It had lost none of its power.

Pacific-wide, this tsunami
cost more than 2,000 lives

and caused millions of dollars' worth
of damage.

Devastating as it was,

the 1960 event was a turning point
in the study of tsunamis.

It was the first time that scientists
could accurately measure

how the size of
an underwater earthquake

directly affected the size of a tsunami.

And conclusive proof
that a tsunami can travel

thousands of miles across the Earth.

And It was with this Chilean earthquake
that we really could prove

that the, uh, undersea motions
associated with the earthquake

are generating these huge effects.

Now scientists had
the evidence to confirm

that undersea earthquakes were
directly responsible for tsunamis.

The ancient Japanese suspicion
was now scientific fact.

In terms of modern tsunami study,
the 1960 wave was year zero.

MOONEY: The Chilean earthquake
was, you might say, the perfect storm,

it's when scientific understanding
had advanced to the point

where scientists had begun to see
the link connecting everything,

so it's a new science,
we're talking about something

which is really only
less than 50 years old.

There are more tsunamis in
the Pacific Ocean than any other.

So in 2004, the world
was taken by surprise

when one of the largest
recorded tsunamis of all time

took place in the Indian Ocean.

On December 26th, 2004,

Indonesia was rocked by the second
largest recorded earthquake ever,

9.2 on the Richter scale.

Minutes later,

a 90-foot tsunami slammed into
the South-East Asian coastline.

225,000 men, women
and children lost their lives.

The Indonesian earthquake
had as much energy in it

as the total energy consumption
in the United States in one year.

This enormous burst of energy
had been released in just seconds.

Once again,
the world had been reminded

of the Earth's awesome power.

In the last 50 years, scientists were
finally able to confirm a solid link

between earthquakes and tsunamis.

By monitoring the size of the
Chilean earthquake in 1960,

scientists were able to prove
conclusively that earthquakes

triggered these gigantic waves.

By following the path of this tsunami,
they were able to prove

that a tsunami could travel
thousands of miles from its origin.

Monitoring the earthquakes that
cause this incredible devastation

involves looking many miles
underground.

By investigating the power at
the root of these giant waves,

scientists can begin to figure out when
and where these waves may strike next.

These dramatic pictures
of the aftermath

of the 2004 Indian Ocean tsunami
show the havoc a tsunami can unleash.

It's almost impossible to
imagine something like that

happening here in the Pacific Northwest.

But Professor Brian Atwater believes
that events like the 2004 tsunami

could one day happen right here too.

He was intrigued

by early settlers' accounts
of Native American folklore tales

that spoke of great waves
sweeping inland.

They convinced him that
huge, locally generated tsunamis

have struck here before,
and could strike again,

posing a threat to tens
of thousands of people

living on the Pacific Northwest coast.

To find out if he was right, he needed
to uncover evidence of past giant waves

hidden in this landscape.

To be really sure it's a tsunami, though,
he would also have to find evidence

of the earthquake that caused it.

Atwater's starting point is the
Copalis River in Washington State,

just a couple of miles
from the long sandy beaches

that make this area
a thriving tourist resort.

In the banks of this estuary lie
buried thousands of years of history.

This is one of the dirtiest jobs in science.

Hunting for evidence of earthquakes
is a muddy business, but it's worth it.

Atwater has found signs
of a potential tsunami.

There's a clue in this bank
that nature has provided, it's this notch.

And notches like this are common

where tsunamis have
laid out sheets of sand

and then later currents

and... and waves come along

and they pluck the sand grains
out of the bank, but they leave the mud.

Atwater has to dig deeper
to find what he is looking for,

a layer of sand that could have
been swept miles inland by a tsunami.

OK, so now you can see the sand.
What deposited this sand?

Maybe it was a tsunami.

To prove that this was
sand from a tsunami,

Atwater's muddy quest must continue.

He also needs to find proof
that the land here around the river

has moved up or down -
a sure sign of an earthquake.

After some hard work, Atwater
finds what he has been looking for -

clear evidence of both
an earthquake and a tsunami.

This time, there was
a human cost as well.

Here we have evidence
for abrupt lowering of land,

and we also have evidence
for the associated tsunami.

In this case, humans are involved -
this was a fishing camp.

Here you have the remains
of that fishing camp

in the form of fire-cracked rocks
which were...

The rocks were used
to heat water, mainly.

OK, so fishing camp,
overrun by tsunami.

Because the land dropped
after the tsunami,

the tides came in
and covered the fishing camp site

and made sure
that people wouldn't use it again.

The land the fishing camp
was built on was dragged down

during the earthquake.

The tsunami deposited sand
over the remains,

and finally, the tide
covered the settlement with mud,

where it remained undisturbed -
until now.

Atwater finally had
the proof he needed.

His Native American myths
of giant waves were no mere legend.

But what was it
that caused the earthquake?

The prime suspect lay 50 miles
offshore - the Cascadia fault.

Cascadia is a major weakness
in the Earth's crust.

Although the Earth may
seem to be a solid sphere,

beneath the oceans
and continents it is divided

into eight major and many minor
segments known as tectonic plates.

Where they meet, they can grind and
jostle against each other at fault lines,

causing earthquakes.

Geologists had long thought
that the Cascadia fault line

was incapable of generating
a major quake.

But Atwater's investigation
has proved that it was highly active.

The big worry for Atwater
and the thousands of people

who live in this region
is that the Cascadia fault line

bears an uncanny resemblance
to another highly active fault line,

the Sunda Megathrust.

It was an earthquake along
this fault that was responsible

for the Indian Ocean tsunami that killed
nearly a quarter of a million people.

Where we get two tectonic plates
coming together,

such as the case of
the Indonesian tsunami in 2004,

one plate pushes beneath the other plate

and creates lots and lots
of friction and tension

and drags the upper plate down with it,

and that process can take
hundreds of years,

even thousands of years.

It's a very slow process.

But eventually the pressure of this
one trying to push back up again wins,

and it flips like that.

And that creates a megathrust,
a sudden movement of the seabed,

and that's what creates
a phenomenal tsunami.

Two factors made this Sunda
Megathrust earthquake so deadly.

The first was its size.

At factor 9.2 on the Richter scale,

this was the largest
in nearly 50 years.

The second was that it took place
not far below the surface.

BOXALL: When we talk
about a megathrust,

that's really where the seabed
is disturbed dramatically.

Sometimes, if the earthquake
is deep in the Earth's crust,

then you see very little surface
manifestation of that earthquake.

If it's quite close to the surface
or very intense,

then quite often you'll see
the seabed itself moving,

and that's what creates
a powerful tsunami.

Investigating the ocean floor
after the quake

revealed that more than 1,000 miles
of fault line had fractured

and sprung up by 60 feet.

This massive jolt pushed up
billions of tons of water,

enough to cover Manhattan
to a depth of nearly five miles.

The rift zone itself was about
a thousand miles long.

We had this entire stretch
of subsea moving,

which creates a huge wave.

So the whole thing
was a phenomenal size

and certainly one of the biggest
tsunamis in living memory.

Atwater's determined research
showed that the Pacific Northwest

was at risk from this level
of devastation too.

But he didn't want to unnecessarily
alarm the coastal inhabitants

until he had collected
all the evidence he could.

Atwater needed to find out precisely
when this tsunami struck this coastline

to see if there could be more.

He first tried radiocarbon dating the soil
along the Copalis River.

But the result could only take him so far.

They showed that the earthquake
and tsunami occurred

somewhere between 1680 and 1720.

More importantly, Atwater still needed
precise evidence of how big it had been.

But so far, his investigation has
uncovered two extraordinary facts.

By unearthing
the abandoned fishing camp,

Atwater could see that
a Cascadia earthquake here

had caused the land to drop.

The notch in the bank was proof

that this same earthquake
had generated a tsunami.

But what these clues didn't tell Atwater
was just how big the tsunami was.

He had no way of pinning down the size
of the threat to the Pacific Northwest.

His investigation was about
to take an unexpected turn,

with clues coming from not
only thousands of miles away,

but also from hundreds of years ago.

Japan has the oldest record of tsunamis
of anywhere in the world.

Samurai writings told of
a huge tsunami in 1700

that had swept over
the east coast of Japan.

It hit without warning,
and destroyed entire settlements.

Japanese scientists were baffled
as to where this wave had come from.

There had been no earthquake to warn
the villagers to make for higher ground.

The mystery wave was dubbed
an orphan tsunami.

Back in the U.S.,

Brian Atwater's investigation into
the mysterious Cascadia earthquake

and tsunami needed more evidence.

He had no accurate way to pin down
either the size or the date of the event.

All he knew was that it had taken place
sometime between 1680 and 1720.

But Atwater's dates were a revelation
to the Japanese scientists.

Could this event be the birthplace of
their 300-year-old orphan tsunami?

And they said,
"By the way, we have this tsunami

"we've been trying to, uh,
find a home for in 1700,

"so we think your... your earthquake
happened in 1700,

"specifically in the evening
of the 26th of January 1700,

"and it was of magnitude nine."

A Cascadia earthquake that
produced a wave with enough power

to cross the entire Pacific Ocean
to Japan

would have had to be a factor nine
at the very least.

This is roughly equivalent to the
enormous Indian Ocean earthquake.

Earthquakes like this
have so much power

that they can send a tsunami
across an entire ocean with ease.

BOXALL: The amount of energy
involved is very hard to estimate,

and it's hard to put it into sort of terms
that people can understand.

We are looking at the phenomenal
forces of several Hiroshimas,

hundreds of Hiroshimas, in fact.

But tsunamis are not just
a very big wave, they're fast.

The big difference
is the scale of the wave -

it's typically three
or four hundred miles long.

It's also not very high -

when it starts off life,
it's usually about two or three foot high.

But it's moving very fast.

It moves at a speed
determined by the water depth.

The deeper the water,
the faster it moves,

so in the deep ocean, this wave
is moving at over 500 miles an hour.

Deceptively, as a tsunami
speeds through deep water,

it may appear completely harmless
and scarcely detectable.

Close to shore, the wave
becomes a deadly killer.

It is only then that a tsunami's
true power becomes clear.

As the wave gets to shallower
and shallower water,

as it approaches a coastline,
the wave slows down.

The shallower the water, the slower
the wave, so it goes from 500, to 400,

to 300, to 200, much, much slower.

The back of the wave
is still going full speed,

and so the whole thing piles up, and
that's why tsunamis are so destructive.

It is this immense speed and power that
reveals how events here in Cascadia

could devastate
a coastal village in Japan,

how an earthquake in Chile
could decimate Hawaii,

and how the Indian Ocean earthquake

could kill almost
a quarter of a million people.

If a quake like this
happened in Cascadia,

the damage it would do
to the Pacific Northwest coastline

would be catastrophic.

But to be sure about the scale
of this threat,

Brian Atwater has to be 100% certain

that the dates of the two tsunamis
were the same.

After fully exploring the estuary
of the Copalis river,

he found one site that might hold
the information he was looking for -

a ghost forest.

This spruce root marks
the remains of a forest

that includes the ghost forest
behind us,

dropped down into tidewater
during the Cascadia earthquake.

This ghost forest is made up

of the standing dead trunks
of western red cedar,

and they were killed on account
of the land here dropping,

and then tides coming in
and surrounding these trees

and bringing in saltwater.

This area would once have been
covered with a dense forest.

But today, only the bleached trunks
of the rot-resistant western red cedar

remain in place.

When Atwater and expert
tree ring specialists cut them open

and studied the lines of growth inside,

they finally cracked
the 300-year-old tsunami puzzle.

The dates of the Japanese
and Cascadia events

were exactly the same -

January 1700.

The Japanese orphan tsunami
finally had a parent.

Maybe there's a certain
amount of justice to it

that... that a place
that doesn't have written records

has these outstanding
geological records.

The link between the two events
made it certain

that the Cascadia earthquake had been
at least an awesome magnitude nine.

And ominously, it is almost
certainly not the only time

that Cascadia has rocked this area.

Atwater believes he has found proof
of a whole series of tsunamis

stretching back 5,000 years.

Each layer of sand in this sample
represents a separate tsunami.

There are places at Cascadia
where I've seen nine stacked up

in a column about 20 feet long.

Uh, nine buried soils, some of
them coated with little sand sheets.

And... and they, you know,
you... you say,

"OK, it's... it's not a question of if,
but it's just a matter of when."

Atwater's tireless detective work
alerted officials

to the increased tsunami threat.

As a result, the towns
along the Washington State coast

have been able to prepare
for this potential catastrophe.

If a Cascadia quake occurred,
the first waves could arrive here

in just 25 minutes.

Tsunami warning signs line the roads,
and sirens stand ready to warn

of an approaching wave.

The lives of thousands of people are
safer thanks to the work of Atwater,

and to some 300-year-old
Japanese writings.

This is a hazard that shows its face
often enough for us to take precautions,

to fasten the seatbelt against it.

By dating the ghost forest along
the Copalis River to precisely 1700,

Atwater had the final proof that
Cascadia was capable of creating

a Pacific-wide tsunami.

Uncovering the multiple layers
of tsunami debris in the riverbank

dating back 5,000 years
show that monster waves

have struck here many times.

This is an ongoing threat.

Atwater knows that another
earthquake is due here,

but he has no way of
knowing exactly when.

Back in the Indian Ocean,

the site of the world's most
lethal tsunami in 2004,

one man has taken the investigation
of tsunamis to a new level.

He believes that he has found a way
to make the Earth's fault lines

give up their secrets and accurately
predict when the next deadly tsunami

could be on its way.

The idyllic looking Mentawai island chain
in Indonesia hides a violent secret,

one that makes it today one of the
most dangerous places on Earth.

These islands lie directly on top
of the Sunda Megathrust,

south of where the enormous
Indian Ocean earthquake

triggered the 2004 tsunami.

The Sunda Megathrust is one of
the largest fault lines on the planet.

Since it caused the 2004 earthquake,

it has also become
one of the most notorious.

Predicting earthquakes here is tricky,

but Professor Kerry Sieh
has a good track record.

He has successfully forecast two
along the Sunda Megathrust already.

The key to successful tsunami
prediction is to forecast

when and where
earthquakes will strike.

And to do this,
scientists must look into the past.

SIEH: If you want to answer
questions about earthquakes

that only happen
every few hundred years

or few thousand years,

well, you've got to find some...
some geological instrument

that allows you to see
those earthquakes.

Professor Sieh has found
an unusual way

to unlock the secrets of the Sunda
Megathrust's turbulent history.

Corals.

These coral atolls are built
from the limestone skeletons

of millions of tiny creatures.

Each generation builds
on the remains of the last.

Over time, the atoll
gets bigger and bigger.

As long as the corals
remain underwater, they flourish,

but once they're above water, they die.

Earthquakes are responsible
for killing all the coral stranded

above water on this beach.

This beach contains corals
of many different ages.

Altogether, Professor Sieh has
nearly a thousand years of history

at his fingertips.

But to unlock the secret history
the corals contain,

he and his team have to take
a less than delicate approach.

We're looking at a sawcut that we
just made through a coral micro atoll.

And the great thing about this head
is it records a sudden drop

of about a foot and a half down to here.

It died down to here,
because the island rose.

The new low tide is way down here.

Everything that was so bold
as to grow up this high dies.

The shape of the coral records
the fall of the Mentawai islands

as they are literally pulled down
by the Sunda Megathrust.

But, crucially, the corals
also record the moments

when the islands are thrust up out
of the water during an earthquake.

Between quakes, the islands are
once again pulled down by the fault

in a never-ending cycle.

You have to imagine that
rocks actually are elastic.

Take a diving board, the diver...
the diver walks out on the platform

and it... it bends like this,

and then he jumps and he springs up
and he jumps off.

And when he jumps off,
the diving board doesn't stay here,

it doesn't go like... it doesn't go like this,

you know, the diving board
springs back up, it's elastic.

Well, rocks are the same,
rocks are elastic too,

so when the Indian Plate goes down
it pulls the Sumatran section down too,

and then later, it fails.

So it just springs up like a diving board.

By analysing corals all over this beach,

Professor Sieh has discovered
a regular pattern to this cycle.

A major earthquake rocks these islands
roughly every 200 years.

What we have here in Sumatra with
the corals is what I call the Holy Grail

of, uh... of earthquake science,
of... of palaeoseismology,

and that is a long record
that has many cycles in it,

a thousand-year-long history
of earthquakes.

But when the geologist
looked even closer,

he saw that the cycle
was more complex.

When we cut a slab, we can see it
in much more exquisite detail

because we can see
what we call the stratigraphy,

or the... the... the layering

and how the layering relates
to the changes of the tide,

so what we can see over here, then, is
the annual bands of growth, right here,

so there's about ten years between
this earthquake and this earthquake.

Professor Sieh
had discovered a major clue.

The corals record that, not only does
a major earthquake and tsunami hit here

every 200 years, but that
they are always accompanied

by a number of smaller quakes.

This is a cycle within a cycle,
a supercycle.

And by counting back the layers
of growth within the coral,

the geologists can put an exact date
on all of these earthquakes.

We know there's a sequence
in the 1350s, 1370s,

we know there's a sequence
in the 1560s, 1600s... 1600,

we know there's a sequence
1797, 1833.

Those sequences are about 200,
to 200... yeah, 230 years apart.

This is crucial information
for the people of the Mentawai islands,

who have no written history.

But Professor Sieh's work
doesn't stop here.

By uncovering their history
in the corals,

he believes that he can now predict
the future for these islands.

And he's already had some success.

Professor Sieh began his work here
in 1993,

and soon realised an earthquake
was imminent.

The Mentawai islands were about
to start their next deadly supercycle.

MAN: OK! Experienced
an earthquake!

In September 2007, he was proved right,

when an earthquake shook the islands
just enough to generate a small tsunami

that wrecked homes and schools.

History is repeating itself,
exactly as he predicted it would.

A much bigger earthquake

and more dangerous tsunami
could be due any day.

SIEH: One section
hasn't failed since 1797,

so, since George Washington
was President of the United States.

We know we're now in a sequence
of at least three giant earthquakes,

we're expecting another one.

The question is whether
the earthquake and tsunami

will be in the next 30 minutes
or the next 30 years.

Thanks to Sieh's research,

the people of these islands
have had time to prepare.

When the wave comes,
they will be ready.

Earthquakes
are forecastable.

If you... if you have enough information
about how they've behaved

over the last thousand years,
or two or three or four cycles,

you can really make
a significant forecast

that people living in the area
actually can do something about.

Education is key.

Children here are now taught

that as soon as they feel
the shaking of an earthquake,

they should run for higher ground.

Newly built roads snake up
steep hills from waterside villages

to allow rapid escape
from the deadly waves.

I'll bet that young children alive today,
if they... certainly if they live to be 60,

they're gonna see that earthquake.

In fact, I think there's a better than
50% chance that it'll happen in...

...within the next 30 years.

By analysing the shape of the corals
on the Mentawai islands,

Sieh has proved that a major tsunami
cycle starts here every 200 years.

By dating the lines within the coral,
he can be even more exact.

They show that these cycles contain not
just one, but several deadly tsunamis.

The Sunda Megathrust
is the clear culprit for tsunamis here.

But not every tsunami is generated
by an earthquake.

A rarer, different type of wave
is out there - a megatsunami.

Although earthquakes are by far
the most common cause of tsunamis,

there is another source
for these deadly waves - landslides.

And these tsunamis
have the potential to be so big

that they have been called
megatsunamis.

Scientists had long suspected that
waves could be generated in this way,

but conclusive photographic proof
wasn't available until 1958.

A landslide into Lituya Bay
in Alaska triggered a wave

that reached heights
of several thousand feet.

This footage,
shot just after the tsunami struck,

shows the wave's enormous power.

The trees here once stretched all
the way down to the shores of the bay,

but were ripped off the slopes
by a wall of water,

leaving nothing but bare exposed rock.

The tsunami was generated
when a relatively small earthquake

triggered a single enormous landslide

of rocks and debris into the bay.

The resulting wave was higher
than the Empire State Building

and stunned scientists
around the world.

Tsunamis on this scale
are incredibly rare.

But another megatsunami,

triggered by a rockfall 10,000
times bigger than Lituya Bay,

could be on its way from a small island
across the Atlantic Ocean.

The Canary Islands,
off the coast of Africa,

are formed from a series of volcanoes.

The youngest is the island
of La Palma.

It is formed from two volcanic ridges.

The first is the extinct Cumbre Nueva
to the north of the island.

The younger, active Cumbre Vieja
lies to the south.

It erupted as recently as 1971.

Geologist Dr Simon Day's
research was crucial

in developing the La Palma
megatsunami theory.

It began with an unusual rift

that had opened up during
a major volcanic eruption in 1949.

We're standing here in the fault
and it runs way down to the south

along the crest of the volcano
for two and a half miles,

so it's one
continuous long structure.

Day believes this fault is evidence
of a geological time bomb,

the beginning of a giant landslide.

What we see here to my right
are layers of... of volcanic rocks,

volcanic blocks here
and layers of volcanic ash.

And on the west of the fault,
we see the same layers of blocks

and ash and those, before
the fault moved, were joined up

and then when the fault moved,
they were separated

and the rocks to my left
moved down and to the west.

What we think will happen
in some future eruption

is that this fault will have gotten bigger

and the whole of this western side
will slide away in a giant landslide

into the ocean to create the tsunami.

This landslide would send the entire
southwest section of La Palma,

one sixth of the island's total mass,
crashing into the Atlantic Ocean

in a single giant landslide.

What we envisage
is the whole of this coastline

and the slope extending up all the way
to the crest of the volcano

that is now in the clouds, all of
that mass of rock would slide away

in a single massive landslide
into the ocean

and pushing the water up in front
of it to create the tsunami wave.

Initially, this wave would be
over 30 times bigger

than the 2004 Indian Ocean tsunami,
more than 3,000 feet high.

(EXPLOSION)

The 1980 eruption of Mount St Helens
was proof that a volcano could collapse

in this terrifying fashion.

This was impressive, but the
collapse of the Cumbre Vieja

would be 200 times the volume of this.

(LOUD RUMBLING)

1,200 billion tons of rock would hurtle
towards the ocean at top speed.

The resulting wave would head
straight out into the Atlantic.

DAY: That wave, of course,
would then spread out

and separate out into smaller waves,

but even so, after crossing the Atlantic
and piling up again on,

for example, the eastern seaboard of
the United States or in the Caribbean

or in northern Brazil,
the waves there, we predict,

would still be between
30 and 100 feet high.

So that's as large as, if not larger,

than the tsunami
that struck Sumatra in 2004.

Boston...

New York...

...and even Miami could all be
under threat from the giant waves.

This was a bold prediction.

Day needed more evidence
to back up his theory.

As he was about to see,
the rift in La Palma's landscape

was far worse than he expected.

The 1949 eruption had left a different
type of geological scar on the island.

Evidence of a more serious weakness
within La Palma

came from a series of eerie-looking
lava flows dotted across the island.

One of the characteristics of
the 1949 eruption that's unusual

is that, instead of starting at one vent
and just continuing there,

a series of volcanic vents opened up
in different parts of the island.

When Day plotted these
weaknesses on a map,

he came to a frightening conclusion.

The rift was far bigger
than he had first suspected.

The area that's potentially affected
is very much greater

than the length of the fault at
the crest of the volcano would indicate,

extending out, um, 10 or 15 miles
from the crest out to sea.

This growing body
of evidence proved

that the rift wasn't just a mere crack
in the surface of La Palma,

but a deep fissure
that reached hundreds of feet

down into the island's foundations.

It is La Palma's volcanic heritage
that is the key to this tsunami threat.

The big hazard here
isn't the eruptions themselves,

it's the fact that the volcano is
building up and building up over time

and becoming more and more unstable,
so that will eventually lead to a collapse.

And it seems that this is not
the first time a La Palma eruption

may have triggered a giant landslide.

Proof lies in the north of the island
in these sheer cliff faces,

formed 65,000 years ago.

What we see in the north of
La Palma is the landslide scar left

when the old volcano
in the north of La Palma

experienced a giant collapse

and produced a giant landslide
off to the west.

So that was a huge collapse -

it removed as much as
100 cubic miles of rock

and deposited it out into the ocean,
so it's the sort of event

that we think is going to happen again

in the future
at the... at the Cumbre Vieja.

This ancient collapse of the
old Cumbre Nueva volcano

is almost certain to have generated
a gigantic wave.

And the next collapse
might not be that far away.

This tsunami could strike in our lifetime.

DAY: Even though
it seems so extraordinary

when we consider it in human terms,

and we talk about a tsunami striking
the east coast of North America

and causing huge devastation
on the scale of the Sumatra tsunami,

but this is what happens
in the geological record,

this is what Earth does.

Although tsunamis have been
documented for thousands of years,

it is only in the last century that
geologists have been able to prove

how they are connected
to the movements of the Earth.

By analysing data from the great
Chilean earthquake of 1960,

scientists were finally able to firmly link
earthquakes with tsunamis.

Unearthing buried
Native American settlements

proved that the Cascadia fault line
in the Pacific Northwest

was an active tsunami threat.

Corals in the Indian Ocean proved that
some earthquake-generated tsunamis

follow a pattern, and strike the
same area with regular intervals.

And the giant rift in La Palma's
landscape shows that tsunamis

generated by landslides are also
a very real threat, megatsunamis,

which could prove to be the biggest
waves that threaten our coastline.

Tsunamis are an inevitable part

of Earth's dynamic structure.

Their capacity to destroy

is awesome,

but, as
scientists

begin to understand more

about the origins of tsunamis,

they are coming closer to predicting

where and when
these monsters may strike.