♪ ♪ NARRATOR: Our planet is capable of unleashing extreme chaos.

Volcanoes, earthquakes, hurricanes, and floods can cause untold devastation.

(people yelling) We may think we've seen the worst Mother Nature can throw at us, but scientists struggling to understand these disasters are discovering evidence that even more extreme events have struck in the past.

JEAN-CHRISTOPHE KOMOROWSKI: So this is about 13 times more powerful than the Pompeii eruption.

NARRATOR: They're uncovering clues that the worst catastrophes in history could strike again.

♪ ♪ Thousands of years ago, floods of unimaginable violence.

This water came up 800 feet.

That's huge.

NARRATOR: Floods powerful enough to blast through miles of solid rock in just hours.

But how?

VIC BAKER: Everything in this landscape was screaming in terms of its signs or clues that this was made by catastrophic flooding.

(water rushing) NARRATOR: The clues to some of the biggest floods ever are here, carved in mysterious rock formations, buried beneath the waves, or hidden in plain sight, all around the world.

♪ ♪ Now, scientists find new clues to understand our volatile Earth... ♪ ♪ (small rocks falling) And unravel the secrets of "Killer Floods," right now, on "NOVA."

♪ ♪ (water rushing) ♪ ♪ NARRATOR: Floods.

Events of such violence, they turn oceans, rivers, and lakes into devastating walls of water.

On average, around the world, these powerful surges kill 25,000 people every year.

In 2004, a deadly tsunami hits Southeast Asia, leaving over 200,000 people dead, and $10 billion worth of damage in its wake.

More recently, in 2017, Hurricane Harvey slams into Houston.

Heavy rains cause catastrophic flooding, killing more than 70, and leaving tens of thousands homeless.

That same year, a third of Bangladesh is submerged by flooding which extended throughout South Asia, including Nepal, India, and Pakistan.

The flooding, caused by an especially strong monsoon, is thought to be the most severe in the last hundred years.

But could Mother Nature have unleashed floods that were even bigger and more destructive in the past?

That's what a series of discoveries is suggesting.

Scientists are unearthing what looks like the scars of cataclysmic floods that dug deep into the rock, reshaping the surface of the Earth itself.

MIKAEL ATTAL: It completely changed the face of the landscape.

BAKER: No one has ever witnessed anything even close in scale.

NARRATOR: Across the world, three far-flung locations share an eerie similarity.

In the United States, 16,000 square miles of dry canyons and bizarre rock formations cover the Northwest.

(water rushing) In Iceland, a 300-foot-deep gorge appears to have been ripped out in an instant.

And off the coast of Britain, a network of mysterious canyons carved deep into the sea bed could reveal how this channel first separated what is now Britain from France.

Far from eroding gradually, there's evidence that vast deluges tore out these landscapes in the geological blink of an eye.

But what could have triggered such killer floods?

And could one strike again?

(water rushing) ♪ ♪ The trail of clues starts here, on the plains of Washington state.

A flat expanse, stretching for hundreds of miles, until suddenly, the landscape changes.

Flat fields give way to sheer gorges, some almost a thousand feet deep.

Rock islands rise to the height of 30-story buildings, while in other places, strange round depressions, like gargantuan potholes, plunge 50 feet.

These are the Scablands, named by settlers who thought the formations resembled scabs or wounds on the rocky terrain.

♪ ♪ Located over a hundred miles east of Seattle, this mysterious landscape covers an area around 16,000 square miles.

(car running in distance) For over a century, geologists have been trying to understand what forces created the Scablands.

BAKER: When you encounter a landscape, it's not unlike a detective encountering a crime scene.

In the case of this landscape, there are features that act like clues.

♪ ♪ NARRATOR: But spotting those clues takes a trained eye.

(helicopter whirring) ♪ ♪ BAKER: You can't really get a sense of this area unless you get up high.

♪ ♪ This part of the Scablands is, like, 30, 40 miles across.

♪ ♪ It's on a mega scale.

♪ ♪ NARRATOR: It's the kind of scale that first led geologists to suspect that the Scablands had formed slowly, eroded over millions of years by rivers, wind, or ice.

(ice grinding) During past ice ages, as temperatures plummeted, giant ice sheets and glaciers carved deep valleys through solid rock.

♪ ♪ Like these, in Glacier National Park, in Montana.

♪ ♪ And rivers, scouring rock over eons, helped carve some of the most dramatic landscapes on Earth, like the Grand Canyon in Colorado.

♪ ♪ But mapping sediments left behind by the ice sheet when it melted 12,000 years ago, shows the ice only made it to the northern edge of the Scablands.

♪ ♪ And Vic Baker's bird's eye view reveals that fast-flowing water was the culprit.

♪ ♪ The clue?

This curved canyon.

BAKER: Looking at this from the air, you can see that the shape is like a horseshoe, which is what forms in waterfalls.

NARRATOR: Niagara Falls and many big waterfalls have a similar horseshoe shape.

♪ ♪ For that reason, this canyon is called the Dry Falls.

♪ ♪ But stretching three and a half miles, this formation is five times the span of Niagara and twice as tall.

BAKER: The cliffs behind me are 400 feet high.

Niagara Falls would fit just within the alcove here.

There's a similar-sized alcove and there's an even bigger one that extends many miles to the east.

♪ ♪ NARRATOR: These are the most extensive falls, wet or dry, known today.

And in a valley below, Vic finds another clue that vast amounts of water once flowed here.

♪ ♪ Features that look like sinkholes, or potholes, often found along the bottom of turbulent rivers.

But these potholes are super-sized.

♪ ♪ BAKER: You can see it's maybe 50 feet deep or so.

♪ ♪ Potholes you see in a normal river are about the size of a person.

♪ ♪ Whereas this would hold multiple elephants.

♪ ♪ NARRATOR: To Vic, all the evidence points to flowing water on a massive scale.

But there is no water flowing here now.

♪ ♪ Today, the largest rivers in the region are the Snake and the Columbia.

♪ ♪ But could they have played a role?

BAKER: The Columbia River lies about 30 miles to the north and isn't big enough to make this kind of feature.

♪ ♪ NARRATOR: The Snake River, as well, is simply too small to carve out potholes and waterfalls on this scale.

To get such an immense volume of water so fast, we need something spectacular to happen.

NARRATOR: So where did the water come from?

To answer this question, scientists are looking at a distant landscape 3,500 miles away.

♪ ♪ Iceland.

♪ ♪ This island, on the edge of the Arctic Circle, is a land of fire and ice.

♪ ♪ (water rushing) In its northeast corner are scars on the landscape that bear a striking resemblance to the Washington Scablands.

♪ ♪ Sheer cliffs over 300 feet high.

♪ ♪ A towering rock island.

♪ ♪ This is the Ásbyrgi Canyon.

♪ ♪ And like scientists in the Scablands, geomorphologist Mikael Attal wants to understand how it was created.

♪ ♪ This really looks like a dry waterfall.

♪ ♪ It's as if there was a big waterfall here and it's not there anymore.

♪ ♪ NARRATOR: Its so similar to the Dry Falls in the Scablands, Mikael suspects they were formed in the same way.

♪ ♪ But how long did it take?

♪ ♪ To find out, he's using a relatively new technique called surface-exposure dating.

♪ ♪ Earth's surface is constantly bombarded by cosmic rays from outer space.

Rocks buried in the Earth are sheltered from these rays, but as soon as the rocks are exposed, like in these cliffs, cosmic rays collide with atoms at their surface.

(water rushing) The force of these collisions knocks neutrons and protons out of the atoms and changes the elements in the rocks.

This leads to the formation of new elements, including a rare form of helium.

These rare helium atoms build up over time, at a predictable rate, so by measuring their concentration, it's possible to determine how long the rock has been exposed.

It's like starting the stopwatch.

♪ ♪ NARRATOR: Mikael samples rocks from all over the Ásbyrgi Canyon and compares the dates when they were exposed.

His results revealed something surprising.

♪ ♪ All the rocks in this area were exposed at the same time, meaning that this entire canyon was carved out all at once.

ATTAL: This canyon was created in one event 9,000 years ago.

(wind howling) NARRATOR: A slow-moving force, like a glacier, erosion, or gradual uplift, would have exposed the rocks along the canyon at different times.

♪ ♪ So it had to be a fast-paced natural disaster, like a titanic flood.

(water rushing) ATTAL: It would have been a flood on a scale far greater than anything that we have witnessed in human history.

(water rushing) NARRATOR: Thousands of miles away, scientists in the Scablands had zeroed in on the same idea.

♪ ♪ BAKER: Everything in this landscape was screaming, in terms of its signs or clues, that this was made by catastrophic flooding.

♪ ♪ NARRATOR: A flood big enough to carve these vast landscapes seems impossible, but flowing water can be surprisingly powerful.

(man yelling in distance) The physical impact of a flood rises with every increase in volume, speed, or duration, and it doesn't take a lot to pack a punch.

A flood just six inches deep can knock people right off their feet, and a flow of just seven miles an hour can have the same force as a tornado.

♪ ♪ (water rushing) But is it possible to blast through solid rock?

Could a flood carve out enormous features like these?

(water rushing) The most destructive floods, from Hurricane Katrina in 2005 to floods in Colorado in 2013, have etched painful memories.

But they have done little to make a mark on the underlying bedrock... ♪ ♪ Like we see here in Iceland and the Scablands.

BAKER: No one has ever witnessed anything even close in scale.

♪ ♪ ATTAL: We're talking about floods here that completely changed the face of the planet.

♪ ♪ NARRATOR: Floods powerful enough to carve whole canyons out of bedrock are rarely seen.

♪ ♪ But in 2002, one was finally caught on camera.

(sirens blaring) On July 4, after a severe storm struck central Texas, Canyon Lake Reservoir flooded, overtopping its dam.

At its peak, enough water to fill an Olympic swimming pool poured over every two seconds.

And when the floodwaters subsided, they revealed a brand-new gorge, carved into the rock 23 feet deep and more than a mile long.

This provided proof that floods can transform whole landscapes in a matter of days, as long as there's enough water flowing quickly enough to produce the necessary force.

(water rushing) (car tires grinding surface) And in Iceland, geophysicist Magnús Gudmundsson thinks he's figured out how such a massive release of water could occur.

♪ ♪ He's come a hundred miles south of Ásbyrgi to the Vatnajökull ice cap, the largest glacier in Europe, similar to glaciers at the end of the last ice age, 9,000 years ago.

GUDMUNDSSON: This glacier is the only possible source of water to create these floods that made Ásbyrgi.

♪ ♪ NARRATOR: In places, it's 3,000 feet deep, hundreds of cubic miles of water locked up as ice.

GUDMUNDSSON: We have all this ice here, but how does it become a flood?

NARRATOR: Magnús believes the secret lies in what's hidden beneath the ice cap: seven huge volcanoes.

In 1996, one of these, the volcano called Grímsvötn, erupted, triggering the most catastrophic flood in Iceland for nearly a century.

GUDMUNDSSON: From time to time, we have these very large eruptions that melt enormous amounts of ice in a matter of hours.

(water rushing) NARRATOR: Almost a cubic mile of meltwater from the eruption tore across the landscape at 16 times the rate of Niagara Falls, destroying roads, bridges, and power lines.

♪ ♪ So today, Magnús is monitoring the volcano.

♪ ♪ By recording elevation and movement, his team has discovered there is a lake of meltwater beneath the ice.

GUDMUNDSSON: You're actually standing on an ice shelf floating on the lake.

♪ ♪ NARRATOR: Magnús now believes that 9,000 years ago... (volcano erupting) ...a giant eruption under the ice cap unleashed a colossal flood powerful enough to carve the Ásbyrgi Canyon.

♪ ♪ So could a chain of events like this have also triggered a flood massive enough to carve the Scablands in North America?

(water rushing) ♪ ♪ Just a hundred miles west of the Scablands are many active volcanoes, notably Mount St. Helens, partially covered in ice.

♪ ♪ (erupting loudly) In 1980, Mount St. Helens erupted, releasing an enormous amount of heat, enough to melt the ice around its crater and trigger dramatic floods.

♪ ♪ But even if all the ice on this volcano had suddenly melted... (water rushing) It would not be enough to carve out rock over a 16,000-square-mile area.

♪ ♪ But what about in the past?

To find out if ancient ice were to blame, Vic Baker needs to find out when the landscape was created with the technique of surface-exposure dating used in Iceland.

(tapping) Taking samples from all over the Scablands, he discovers most of the rocks were exposed within a few thousand years of each other, and one date in particular stands out.

♪ ♪ BAKER: Many of the dates we get are in the range of about 16,000 years ago.

♪ ♪ NARRATOR: Although ice sheets covered much of North America 16,000 years ago, geologists believe the ice stopped short of the volcanoes of Washington state.

♪ ♪ The trail of clues seemed to dry up... ♪ ♪ ...until a surprising discovery 200 miles east of the Scablands, in Missoula, Montana.

♪ ♪ Here, geologist Larry Smith is heading up into the hills, where he sees a series of horizontal lines a thousand feet above the valley floor.

SMITH: They look very much like they'd have been cut into the hillside by waves beating against the rock.

These lines are clearly lake shorelines, and show that an immense body of water temporarily filled these now-dry valleys of western Montana.

♪ ♪ NARRATOR: Tracing these ancient shorelines for hundreds of miles, geologists have calculated that these valleys were once filled by a body of water larger than Lake Ontario.

♪ ♪ SMITH: When there was a lake here at 4,200 feet, we would have had a beach right here in front of us, or a shoreline, and extending all the way across to the other side of the valley, with a thousand feet of water over what is now the city of Missoula.

NARRATOR: Geologists call it Glacial Lake Missoula.

And despite being 200 miles away from the Scablands, Larry suspects it held enough water to tear through the area.

♪ ♪ That is a vast amount of water, and if this lake drained very rapidly, it would be fundamental to carving the channel in Scabland.

♪ ♪ NARRATOR: But today, there is no lake here because the valley is open-ended.

♪ ♪ So where are the formations that held the water in place?

♪ ♪ SMITH: So the question is, is where, when, and how did a dam form to create this lake?

♪ ♪ NARRATOR: Searching for clues, Larry travels back down to Clark Fork, at the narrow end of the valley.

He sees no signs of landslides or rock falls that could have dammed the lake in the past.

But on the bare rocks, he spots some tell-tale markings.

SMITH: You see scratches within the rock, geologically it is impossible to smooth off rock and scratch it without glacial ice.

♪ ♪ NARRATOR: As the glacier moves, rocks embedded within it scratch the bedrock like sandpaper.

So these scratches are evidence that during the last ice age, a glacier moved across this valley.

And by mapping where rocks have been scratched, geologists have discovered that the Clark Fork River Valley was once blocked by a giant finger of ice 23 miles wide and half a mile deep.

Larry Smith believes this ice dam created Lake Missoula.

SMITH: It blocked the drainage of the Clark Fork River.

The water had nowhere else to go, so backed up a lake behind this large glacier in this valley.

♪ ♪ NARRATOR: All the evidence points to a massive reservoir of water held in place by a giant dam of ice, a lake large enough to have carved out the canyons of the Scablands... (water lapping gently) ...if it were released in one catastrophic event.

(water rushing) ♪ ♪ (birds chirping, insects buzzing) The idea that there had once been a lake here that had suddenly drained also explains one of the other striking features of the valley floor-- giant ripples.

♪ ♪ SMITH: These straight crested hills are current ripples that show water flowing from where we're standing off to the distance.

♪ ♪ NARRATOR: Ripples like this are made by flowing water, like the tide moving in and out on a beach.

The faster the flow of water, the larger and more widely spaced the ripples become.

Here, they're giant things that are spaced hundreds of feet apart and they're tens of feet high.

NARRATOR: These ripples are so high, the lake water that created them must have poured through this valley at speeds of up to 80 miles an hour.

♪ ♪ (water rushing) It's evidence that Lake Missoula was unleashed rapidly in a massive flood.

♪ ♪ But that means the 23-mile-wide ice dam holding it in place must have suddenly given way.

(ice crumbling) How could an ice dam of this scale fail so catastrophically?

♪ ♪ The exposure dates of the rocks in the Scablands reveal the flood occurred well before the end of the Ice Age.

This rules out gradual melting from a warming climate.

♪ ♪ Could looking at modern dam failures hold a clue?

♪ ♪ MAN (over radio): You people down the stream better get out.

NARRATOR: In 1976, the newly constructed 300-foot-high Teton Dam, in Idaho, failed, unleashing almost 80 billion gallons of water.

♪ ♪ Investigators discovered that water had seeped under the earth-filled dam, eroding it from below.

Larry Smith believes water seeping under the ice dam also caused the catastrophic release of Lake Missoula.

♪ ♪ SMITH: At the bottom of this 2,000-foot-deep lake, the water pressures are immense, and any small cracks in the ice will get penetrated by that high-pressure water.

In doing so, that'll expand that crack network to form tunnels under the ice.

(water flowing) NARRATOR: Lake water began draining through these tunnels at a faster and faster rate... (water rushing) ...until the whole ice dam suddenly collapsed.

(cracking loudly) (water rushing) SMITH: It falls within minutes to hours, with a cascade of water coming through the area.

(wind howling) NARRATOR: All signs point to Lake Missoula being the source of a catastrophic flood.

♪ ♪ Still, how likely is it that floodwater could travel hundreds of miles southwest to the Scablands with enough power to carve out solid rock and transform the entire landscape?

♪ ♪ ♪ ♪ Roger Denlinger studies fluid dynamics.

He's taken the volume of ancient Lake Missoula and 3D maps of the Scablands to build a computer model that will predict where the ancient flood would have traveled.

Effectively, you're just pouring water over the landscape.

This is simply water flowing over the Earth's surface, and it's going to always head in the direction that it sees as downhill.

♪ ♪ NARRATOR: Roger's model will also determine the depth of the water.

And this color bar shows the flood's erosive power.

♪ ♪ If flow lines in the model turn red, Roger knows the water was flowing with enough force to carve solid rock.

At this point, we're going to break the dam.

♪ ♪ (ice cracks sharply) (water rushing) NARRATOR: The moment it's released from the ice dam, the lake water rushes southwest, toward what we know today as the eroded landscape of the Scablands.

♪ ♪ And not only that, the places that the model has highlighted in red, where the power of the flood is greatest, exactly match the location of the most dramatically transformed landscapes today: ♪ ♪ the Dry Falls; ♪ ♪ the rock islands, ♪ ♪ and the sheer gorges.

♪ ♪ We get damage to the surface in exactly the areas that we see today.

♪ ♪ NARRATOR: Roger's model supports the theory that a giant flood from Lake Missoula carved this landscape, and reveals that the waters reached unimaginable heights.

This water came up 800 feet-- that's huge.

(water crashing) BAKER: Most people think of floods by watching the TV and they see the water rising in a river, and they see a house going underwater, maybe there's a person on top of the house.

Think of water hundreds of feet above the house, that's the difference in the scale of this flooding.

NARRATOR: Bringing all the evidence together, scientists can now unpack the catastrophic flood blow by blow.

Around 16,000 years ago, the vast ice dam holding back Lake Missoula failed, suddenly unleashing 500 cubic miles of water.

(cracking loudly) (water rushing) SMITH: It was equivalent in volume to ten times all the rivers of the world's natural flow.

NARRATOR: The raging torrent tears across Washington state, ripping out billions of tons of rock from the once-flat landscape.

BAKER: There would be blocks of ice, there would be boulders, there'd be roiling water, the sound would be overwhelming.

NARRATOR: In a matter of hours, the flood reaches the Pacific Ocean, carrying with it 1,200 cubic miles of rock and earth, violently torn from the Scablands.

♪ ♪ SMITH: To do all this landscape change within a few days to a few weeks is just mind-expanding.

♪ ♪ BAKER: Even Hollywood disaster movies do not compare to what would have happened as this flood came across the landscape.

(water rushing) ♪ ♪ NARRATOR: This dramatic event entirely reshaped the landscape.

But there is one final twist in the tale.

♪ ♪ Further research, based on core samples drilled out from the floor of the Pacific Ocean, suggests that the Scablands are a product not of one, but of many floods.

The evidence reveals that during the Ice Age, beginning around 20,000 years ago, repeated floods tore across the landscape... ♪ ♪ ...as the giant ice dam repeatedly broke, reformed, and then broke again.

♪ ♪ BAKER: The circumstances that created this immense volume of water produced multiple floods.

(water rushing) ♪ ♪ NARRATOR: Decades of geological detective work show that the scarred and eroded landscapes of Washington state, as well as Iceland, both bear the fingerprints of mega-floods.

And now, this discovery is helping scientists unravel a mystery in another part of the world.

♪ ♪ Thousands of miles away is the channel that separates what is now England from France.

♪ ♪ Today, it links the North Sea in the east to the Atlantic Ocean in the west.

Called the English Channel, it's the busiest shipping lane in the world.

♪ ♪ And towering more than 350 feet above it, on the south coast of England, are the White Cliffs of Dover.

♪ ♪ Geologists, like James Lawrence, now think these iconic chalk cliffs hold an extraordinary secret, and he's going over the edge to hunt for the evidence.

♪ ♪ Because these cliffs look almost identical to cliffs on the other side of the channel, on the northern coast of France.

(seagulls squawking) LAWRENCE: People don't realize that if I was to go over to France, we could find similar chalk cliffs.

NARRATOR: This chalk formed 100 million years ago, when this whole area was covered by a tropical sea.

(waves lapping) The ancient sea teemed with microscopic organisms.

When they died, their calcium-rich skeletons fell to the sea bed.

Over time, these built up in thick layers and were compressed into chalk, a kind of limestone.

LAWRENCE: We are getting exactly the same rocks which have been deposited in exactly the same environment on this side of the channel, and on the French side of the channel.

(waves lapping) NARRATOR: And James is discovering that the connection between the cliffs in France and England goes beyond the chalk itself.

♪ ♪ Embedded in the white chalk are a series of horizontal bands of a dark rock called flint.

Here I have a fantastic band of flint.

♪ ♪ NARRATOR: Flint, a form of the mineral quartz, is formed by changes in ocean chemistry.

♪ ♪ But these changes occur only occasionally, resulting in these distinctive dark bands.

♪ ♪ LAWRENCE: These flint bands are continuous throughout the chalk.

♪ ♪ NARRATOR This band of flint runs through the entire cliff, and there are dozens running horizontally... ♪ ♪ ...each one at a different level in the chalk.

Taken together, these parallel bands of dark flint form a unique geological fingerprint in the white cliff.

♪ ♪ What's extraordinary is that the same geological fingerprint is visible on the other side of the channel.

(seagulls squawking) So the chalk and the flint in these cliffs forms a bar code and is exactly the same as the chalk and the flint in the cliffs in France.

NARRATOR: The spacing and levels of the flint layers perfectly align.

♪ ♪ To James Lawrence, this raises an extraordinary possibility.

So what we know from this evidence is that a chalk ridge once connected England and France.

NARRATOR: These flint layers tell us that hundreds of thousands of years ago, a ridge of chalk almost seven miles wide once extended 21 miles across the channel, joining what is now Britain to the European continent.

♪ ♪ LAWRENCE: So it's quite incredible to think that there would have been a land mass stretching across the sea.

(seagulls calling, wind blowing) NARRATOR: But this discovery raises a brand-new mystery.

(chalk pieces falling) Somehow, the cliffs between England and France have been separated over time.

♪ ♪ NARRATOR: If Britain and France were once joined, what force separated them and turned Britain into an island?

♪ ♪ Control, Maverick.

♪ ♪ NARRATOR: While exploring the sea bed of the English Channel, geologist Jenny Collier finds a telling clue.

♪ ♪ Four, five, six-- ten meters in a split second.

We've got a really steep drop-off in the topography and it's the edge of a really unusual landform.

NARRATOR: Using sonar to measure the depth of the channel, Jenny is surprised to find what appears to be a steep canyon carved into solid bedrock.

♪ ♪ Sonar works by firing sound waves at the sea bed.

The deeper the water, the longer it takes the sound to make the round trip.

♪ ♪ Jenny expected the channel floor to be flat, but the sonar has revealed something far more dramatic.

COLLIER: We've discovered just an extraordinary geological event, right in the middle of the straits.

♪ ♪ NARRATOR: To learn more about this major geological find, she and her colleagues took on a massive task.

♪ ♪ Using a more advanced sonar system, they are mapping 53 square miles of the channel, to an accuracy of four inches.

(sonar beeping) What this reveals is a strange picture of channels, rock islands, and valleys, carved nearly 300 feet down, into the rock of the sea bed.

♪ ♪ I mean, we haven't got anything like this in Europe.

There's really only one place that has all of these features.

♪ ♪ NARRATOR: Without the water, the landscape beneath the English Channel appears to have steep valleys and islands carved into it.

It looks eerily similar to the channeled Scablands of Washington state.

But was this underwater landscape also created by a mega-flood?

COLLIER: The only way you can dig out islands into solid bedrock is to have extreme water flows, and that basically pointed us towards, this was yet another catastrophic flood terrain.

(water rushing) NARRATOR: What the Scablands revealed is that carving solid rock requires a huge reservoir of water to be trapped, then released in a single cataclysmic event.

♪ ♪ But today, the English Channel flows between two open seas.

So how could a large enough volume of water have built up to cause a mega-flood?

♪ ♪ Geologist Phil Gibbard believes he has an answer.

And the evidence lies 120 miles north of the English Channel, on the coast of the North Sea, at the bottom of these cliffs.

GIBBARD: What we've got here is a glacial deposit which is from about 450,000 years ago.

♪ ♪ NARRATOR: Deep, fine-grained deposits like this were laid down across Northern Europe as giant ice sheets ground over rocks.

450,000 years ago, England was in the grip of an ice age.

(ice grinding) Ice sheets, hundreds of miles across and a mile high, reached down from Scandinavia.

They would have dammed the northern edge of the North Sea.

To the south, the intact ridge of chalk between what is now France and England formed a natural dam.

Phil believes that meltwater from the ice sheets and rivers pouring into the North Sea had nowhere to go.

A vast amount of water built up behind the chalk ridge.

He sees the evidence for this ice age reservoir in the sea cliffs, as thin horizontal layers of silt.

GIBBARD: The sediments are horizontal, as you see.

That horizontality can only be produced in a lake situation, a standing-water situation.

NARRATOR: And not in a turbulent area, like an ocean.

Phil has discovered similar-looking formations in other places around the North Sea, some a hundred feet above sea level today.

♪ ♪ GIBBARD: So this was a massive lake on the scale of the Great Lakes in North America, and this lake provides the only possible source for the mega-flood that formed the Dover Straits.

NARRATOR: Could this enormous reservoir, a glacial lake, have suddenly drained to form the dramatic features on the bed of the English Channel?

And if so, how?

♪ ♪ COLLIER: In order to carve these features, this rock ridge must have failed very, very rapidly.

♪ ♪ NARRATOR: But what could have caused this?

♪ ♪ How could the giant ridge of solid rock between France and Britain have given way so catastrophically?

(waves lapping) A clue lies in the way chalk reacts to water.

♪ ♪ LAWRENCE: Having a glacial lake in contact with a chalk ridge would have saturated the chalk, making it much weaker and much more likely to fail.

NARRATOR: When water soaks into chalk and saturates it, the chalk can lose half its strength, making it far more likely to fail.

One of the problems with the chalk being so weak is that it will often lead to cliff collapses, like the one we can see behind us.

♪ ♪ NARRATOR: Every year, thousands of tons of rain and wave-soaked chalk collapse into the channel, dramatically eroding the coastline.

♪ ♪ Many geologists now believe that during a previous ice age almost a half million years ago, water from the North Sea reservoir soaked the chalk ridge, fatally weakening it.

♪ ♪ Once the lake was deep enough, water began pouring over the top of the ridge in a waterfall, rapidly eroding the waterlogged chalk.

(water rushing) We'd have had initially a small stream of water coming over the top of the rock ridge, that would have catastrophically crumbled, with large amounts of rock being removed and more and more water flooding through, just running away with itself.

NARRATOR: From the shape of the features on the sonar, Jenny estimates that the floodwaters raced through at a rate of about 264 million gallons a second.

♪ ♪ That's almost 60 times the flow rate of the Mississippi River.

(water rushing) You would have seen a tidal wave overtopping and washing a giant gorge into that landscape.

NARRATOR: The deluge crashed on, breaking through the chalk ridge linking today's Britain and France, before finally reaching the Atlantic Ocean.

It was this cataclysmic flow that created the English Channel and began the process of erosion that led to what's now Britain becoming an island for the first time.

♪ ♪ The clues in Iceland, the English Channel, and the Channeled Scablands of Washington state reveal that floods bigger and more devastating than anything we see today have torn across and helped shape Earth's surface.

BAKER: These giant mega-floods totally shaped a landscape in a matter of days or weeks.

♪ ♪ NARRATOR: But the question is: could a flood on this scale happen again?

(ice cracking softly) The one thing all these mega-floods have in common is that they involve huge volumes of ice melting and being released in one sudden burst.

(crashing loudly) In Iceland, a volcano beneath the ice sheet could trigger a mega-flood at any moment.

(volcano erupting) ♪ ♪ Fortunately, very few people live in the Icelandic flood zone, and the huge volume of ice needed to create glacial lakes, on the scale of the ones that carved the English Channel and the Scablands, can only build up during ice ages.

(whirring) ♪ ♪ But there is one region on Earth today where stores of melting ice still pose a major flood risk to millions: ice- and snow-covered mountains.

♪ ♪ SMITH: Wherever you have glaciers, you have a lot of water.

Wherever you have glaciers in a mountain, you have the high likelihood of making a glacially dammed lake, and those glacially dammed lakes are unstable and could drain catastrophically.

♪ ♪ BAKER: We're not going to get, today, releases of water like Lake Missoula, that was 2,000 feet deep.

But we can get glacial lakes that are a hundred feet deep, and these will produce really dangerous and spectacular floods.

♪ ♪ NARRATOR: Today, floods in populated areas wreak untold devastation.

Faced with Hurricane Harvey's impact on Houston and the floods in Bangladesh, it may seem that floods could not get any worse.

But the vast floods of the past carved huge features into the very bedrock of continents.

Those scars are a stark reminder of just how destructive floods can be.

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