Original subtitles

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.