Tsunamis 10 Things You Didn't Know About...

Two thirds of Planet Earth is covered by ocean.

We need this water to sustain life.

But water can also be one of the most destructive forces on this planet.

Tsunamis, or giant ocean waves, have wreaked havoc throughout history.

The forces that create such massive waves are incredible.

What's more, as a geologist,

I find the stories about tsunamis truly astounding.

So, if you don't know which great civilisation

may have been devastated by a tsunami...

how a father and son managed to surf the highest wave ever recorded and survive...

..or why millions of Americans could be at risk from a mega-tsunami,

then stay around as I reveal

10 things you didn't know about tsunamis.

I'm here on La Palma, one of the Canary Islands.

Most people come to the Canaries to get away from it all and relax,

but not me.

I've come here because this little island might one day generate a massive tsunami...

A mega-tsunami.

We'll explore that story later,

but first I've got nine other tsunami stories

to cover from around the world.

We'll begin with one tsunami that's burnt into all our memories,

it's certainly etched into my mind,

and that's the Boxing Day tsunami of 2004

which caused destruction around the Indian Ocean.

On the morning of December 26th, 2004,

a huge underwater earthquake ripped apart the seafloor

northwest of Sumatra, Indonesia.

When it subsided, no-one had any idea

that the tremors had generated something even more deadly -

a tsunami.

A chain of waves fanned out across the Indian Ocean,

travelling at the speed of a jet plane,

hitting coastlines all around the Indian Ocean.

The images of waves and death were unforgettable.

I watched these images,

amazed at how easily people's lives and homes were swept away.

The tsunami had hit the nearest shores with such force

it rose to over 60 feet, and surged inland for up to three miles.

It destroyed everything in its path.

Leaving a devastated Sumatra behind,

the tsunami continued towards Thailand.

SHOUTING

The waves arrived faster than people could run,

hurling them against buildings and trees

and then snatching them out to sea.

Meanwhile, the westbound waves roared across the Indian Ocean,

slamming into Sri Lanka and even as far as the East African coastline.

What's incredible is that the tsunami didn't stop there.

It was so powerful that it travelled around the whole world for 40 hours.

By the end, the Boxing Day tsunami had claimed more than 225,000 lives,

in 11 different countries,

from as far afield as Indonesia and South Africa.

So, just what makes tsunami waves so different

and so much more destructive than ordinary storm waves?

Well, an ocean wave is caused by the effect of the wind on the surface of the sea.

But a tsunami is triggered when a huge volume of water,

not just on the surface but right down to the ocean floor, is shifted in one sudden violent motion.

This rapid movement can happen after a volcanic eruption or a landslide,

or an underwater earthquake.

When an earthquake, for example, cracks the ocean floor,

one side of the fault is thrust up.

This then pushes up the whole body of water above the fracture as well,

creating a wave on the surface of the sea that becomes a tsunami.

In mid-ocean, the ripples of a tsunami have a small wave height

and a very long wavelength.

That's the distance from the front of the wave to the back of the wave

and this can be hundred of miles long.

But as the tsunami reaches land,

it goes through a frightening transformation.

From the shore, the first sign that something is wrong may be the water

along the beach being sucked back toward the source of the tsunami.

This is called "drawback".

Then, as the tsunami itself nears land,

the shallow water acts like a brake,

slowing the front of the wave dramatically,

but the back of the wave, hundreds of miles behind, is still travelling fast.

It now catches up,

causing the front of the wave to rear up into a wall of water.

But there is worse to come. Instead of breaking onshore,

the whole length of the wave sweeps onto land,

engulfing everything in its path.

And that's what happened on Boxing Day 2004.

A few weeks later, I visited Thailand,

one of the worst hit areas, and I was shocked by what I found there.

I guess it was the first time I realised the devastation

that can be caused by a wall of water.

For a geologist like me, it was obvious that some truly

catastrophic geological event had happened to create a tsunami.

And indeed it was caused by a huge earthquake.

But this was no ordinary earthquake.

And to understand just how different this earthquake was,

a group of scientists set off across the Indian Ocean to study it.

They wanted to find actual evidence of the earthquake on the sea floor

to work out how it had caused such a huge tsunami.

What they were to discover on this expedition would shock them.

But first, they would have to actually find the fracture in the Earth's crust.

Below the ship lies a vast undersea chain of mountains,

as high as the Alps.

They've been pushed up over millions of years

by the movement between two giant tectonic plates.

Tectonic plates are slabs of the Earth's crust colliding together,

and they're often the cause of deep ocean earthquakes,

where one plate is pushed underneath the other.

Somewhere below this spot, the sea bed

ruptured for many hundreds of miles on Boxing Day 2004.

This is where the tsunami was born.

Using remote equipment,

scientists are now able to scan the ocean bed in great detail.

Oh, we came down.

BEEPING We are at the bottom.

It didn't take long before they quickly identified

a dramatic geological feature.

The line they had found

looked like it could be a fracture caused by an earthquake.

As they edged forward,

the sonar image revealed something very large, dead ahead.

-Look at the sonar.

-Yeah.

We got a big target in front of us now, about eight metres out.

Oh, my God.

It was a sheer vertical cliff thrust out of the sea bed.

A wall large enough to have displaced a huge amount of water

and the physical evidence of a massive earthquake.

But to be sure this wasn't an ancient fault,

they needed to inspect the top.

If it showed a sharp, saw-toothed edge,

it would prove this was a recent fault.

OK, here we go, here we go up.

And at the top, there it was - a rough, saw-toothed edge,

proof that this cliff had been formed recently.

The scientists had found the Boxing Day earthquake fault.

But no-one was expecting what they discovered next.

As they climbed higher, something else loomed out of the darkness -

a second cliff beyond the first, and this one was enormous.

These two sheer cliff faces were the evidence that proved

to the scientists that this had been no ordinary earthquake.

This was what they call a mega-thrust earthquake.

Unlike ordinary earthquakes, mega-thrust earthquakes

rupture over many hundred of miles and are always over a magnitude 9.

They're the largest earthquakes in the world.

And what most people don't realise,

is that when this type of earthquake happens,

it will create a far bigger tsunami.

Having seen this dramatic evidence of uplift on the sea floor,

the scientists were now able to outline the exact chain of events

that devastated so many lives.

Deep under the Indian Ocean, two tectonic plates

had been pushing against each other for hundreds of years.

The edges of these plates were locked together,

building enormous stresses and bending the upper plate

like a giant spring.

And on Boxing Day 2004,

this pressure reached breaking point.

It happened with such enormous power that it was a mega-thrust event.

The fault started to rupture, shooting upwards by as much as 40ft.

At twice the speed of a bullet,

the plates unzipped over a distance of more than 750 miles.

It lifted the sea bed and the entire ocean above.

Billions of tons of seawater,

forced upward by the movement of the sea bed

now flowed away from the fault.

On the surface, the displaced water fanned out

as a series of giant ripples.

The tsunami began to travel at up to 500 mph.

And so what triggered the Boxing Day tsunami was no ordinary earthquake -

it was a rare mega-thrust earthquake

that had set in motion this wave of destruction.

The violence of the Indian Ocean tsunami showed us just how destructive these waves are.

But this tsunami did one further unexpected thing that had nothing to do with destruction -

it created a moment of revelation.

In one brief instant, the remains of an ancient city

lost just off the coast of India were revealed.

And that's the subject of my next story -

the legend of Mahabalipuram, the lost temples and the tsunami.

Mahabalipuram is a beautiful complex of temples

that sits near an ancient port city on the east coast of India.

Built around the seventh century,

only a precious few of the original temples have stood the test of time.

One of the remaining temples is known as the Shore Temple,

as it sits dangerously close to the coastline.

Local fishermen tell an interesting story about this temple.

The legend says that the Shore Temple is in fact

the seventh temple of a series, called The Seven Pagodas.

These temples are said to have once lined the shore,

and according to the myth,

they were so beautiful that the Gods grew jealous, and they sent a flood

that submerged six of the temples below the waves.

This left only one temple still standing on the coast - the Shore Temple that we can see today.

But this wasn't to be the last we would hear of the six lost pagodas.

On the morning of Boxing Day 2004, the tsunami reached

the East Indian coastline and reared up on Mahabalipuram.

Standing near the beach was a group of fishermen.

They looked on as the sea retreated for up to half a kilometre,

drawing back in the characteristic way just before a tsunami strikes.

In the few short minutes that it took for the sea to retreat from the shore,

tourists and fishermen stared spellbound

by what the receding waters had revealed.

They swore they had seen the six lost temples of Mahabalipuram.

The onlookers claim that for several minutes,

the lost temples had been revealed by the receding ocean.

And then they were gone, as the wave swept up the shoreline

engulfing everything these people had seen.

Leaving everyone wondering, I guess, if they'd dreamt it all.

So no-one is able to say for sure

if the tsunami had revealed the lost temples of the myth.

But I'd like to believe it's true,

that for a few magical moments, those people on the beach

saw the remaining temples of The Seven Pagodas.

Whatever they saw, it's now under the water,

and presumably out of sight for ever.

Our next tsunami takes us way back in time,

back to the age of the dinosaurs.

And it wasn't triggered by an earthquake or any other natural force on THIS planet,

the trigger for my next tsunami...

came from outer space.

Long, long ago, in fact 65 million years ago,

one of the most catastrophic events this planet has ever seen happened.

An enormous meteor struck the earth -

a meteor the size of San Francisco.

This is the meteor that many believe wiped out the dinosaurs

by creating massive climactic change.

But my story isn't about whether or not the meteor wiped out

the dinosaurs or even whether it caused any change in the climate.

The surprising detail that most people don't realise about this meteor strike

is that it must have created a gigantic tsunami.

And that's because when it hit the Earth, it struck not on land but on the water.

It must have been the mother of all tsunamis.

In 1991, scientists discovered the meteor's ancient impact site

near the Gulf of Mexico.

It had struck just off the coast of Mexico's Yucatan Peninsula.

The crater dug into the ocean floor,

was 112 miles across

and 20 times as deep as the Grand Canyon.

The meteor must have been six miles wide.

So what kind of tsunami did this impact cause?

Well, it would've created something very different from an ordinary tsunami.

Travelling at over 700 miles a minute, the meteor impact

would've created an incredible fireball, equivalent to billions of Hiroshima bombs exploding.

The sheer force of the impact will have vaporised the water,

releasing steam that will have blown a hole in the ocean.

The meteor carved out this huge cavity in the sea floor.

And then the sea that rushed in to fill the crater rose up,

creating a colossal wall of water.

When this wall of water then collapsed,

it sent out a very unusual kind of tsunami wave,

as high as the seafloor was deep.

What's different is that it will have created a rather unique chain of tsunamis.

And that's because the process will have continued oscillating inside the cavity,

like the water sloshing around in a bath,

creating not one but many gigantic tsunami waves.

This chain of waves will then have radiated out, hitting coastlines thousands of miles away

and travelling up to 100 miles inland,

destroying everything in its way.

We'll never know how many dinosaurs were killed by the tsunami itself

rather than the climate change that would have followed.

But one thing's for sure - this wasn't just one of the biggest meteor impacts,

this was most likely one of the largest tsunamis ever.

It must have created the most unbelievable spectacle.

And who witnessed it? The dinosaurs.

Our next story takes us to the myth of Atlantis,

the legendary tale which Plato wrote

about a magnificent civilisation which sank below the sea.

Some scientists believe that they've found the island which might've been the inspiration for Plato's story.

And, of course, at the heart of the tale is a gigantic tsunami.

Over 2,000 years ago,

the Greek philosopher Plato wrote a story about a super civilisation

which lived on the mythical island of Atlantis.

In Plato's story, the Atlantians ruled the Mediterranean.

But, "In a single day and night of misfortune",

Atlantis and all its inhabitants were swallowed by the sea.

But could there be a true catastrophic event from history

that prompted Plato to invent the story of Atlantis?

Well, perhaps.

Some scientists now believe that Plato may have been inspired

by what happened to the Minoans on the island of Crete.

The Minoans were the first great European civilisation.

They flourished 5,000 years ago, long before Plato.

They built palaces of the finest architecture

and were ruled by powerful kings.

Minoan artists recorded their lives in stunning frescoes.

What's more, their fleet ruled the Mediterranean.

But about 3,500 years ago, it all came to an abrupt end.

Over a few generations,

this great civilisation was wiped from the pages of history.

For archaeologists, it had always been a complete mystery.

What had happened to the Minoans? No-one could understand why they had disappeared so abruptly.

But then they found an unexpected clue on the hill sides of Crete.

High above sea level, they discovered sea shells.

Wow, look at that!

They found shells in places they should never have been.

They guessed that a tsunami could have put them there.

So they set about looking for the geological event

that could have caused such a tsunami.

They dated the organic matter in the surrounding sediment and came up with a very interesting date -

around 1630 BC.

That's 3,500 years ago.

At this moment, the penny dropped.

Because this date is about the time of one of the most cataclysmic

geological events of the ancient world.

An event we know created a huge tsunami -

the eruption of the volcano in Santorini.

Over the course of a few days, this monstrous volcano

belched ash, gas and rock up to 25 miles into the atmosphere.

Eventually the huge magma chamber collapsed, and the sea rushed in.

The result was an even more cataclysmic explosion,

now considered one of the most explosive volcanic eruptions in history.

All the debris eventually plunged into the sea.

As it hit the water, the sea will have risen up,

creating a massive wall of water.

This colossal wave must have then raced across the sea,

consuming anything in its way.

In only 20 minutes, it will have reached the island of Crete,

lying directly south of the volcano.

The Minoan people will have heard the explosions,

but will have no idea what was heading towards them.

There would have been no chance of escape.

When it hit the shore, it reared up 60ft in the air

and engulfed everything in its path.

It would have instantly drowned tens of thousands of people,

swallowing them up in one enormous wave

then dragging them back out to sea.

It must have devastated the island.

And yet it won't have wiped the Minoans out all together.

There would have been survivors.

Many of them will have run to high ground.

But the wave will have affected them in a more sinister way -

it will have covered the fields in salt water

and the crops will have failed.

The Minoans could not have fished

as the volcano may well have polluted the sea.

Many would have starved and fought with each other to survive.

We know that within a short time of this event,

the Minoans had disappeared completely.

The end of Europe's first great civilisation

devastated by one natural disaster.

So, if the dramatic ending of the Minoans was indeed linked to the massive eruption

at Santorini and the huge tsunami that followed,

then maybe it was the story that inspired Plato to write the tale of Atlantis.

I would say this is as good an explanation, of how an island civilisation

can be lost to the sea, as we will ever get.

So far we've covered stories from around the world,

but actually 80% of tsunamis happen just in the Pacific.

So if you happen to be an island in this vast ocean with these forces

of destruction let loose all around you, you're in deep trouble.

That's why the tsunami capitals of the world are Hawaii and Japan.

Most of us think of tsunamis as being rare random events

that can happen anywhere, in any ocean.

But surprisingly, some places are constantly bombarded by them.

Of all the oceans of the world,

the Pacific has the most frequent tsunamis.

In fact, four out of every five tsunami happen in the Pacific Ocean.

So just why is the Pacific so prone to tsunamis?

Well, unlike all the other oceans of the world, the basin of the Pacific Ocean is encircled

by a zone of frequent earthquakes and volcanic eruptions.

These are the result of the shifting and colliding of the Earth's plates in this particular area.

They call it the Pacific Ring of Fire.

When there's an earthquake or a volcanic eruption along this rim,

it can trigger a tsunami that will then rip across the entire ocean.

And if you're an island in the middle of the Pacific,

you'll be bombarded by all of these waves from every direction.

And that's what happens to one set of islands

right in the middle of the danger zone.

Hawaii may be a paradise on Earth for surfers and sun worshippers,

but they have been struck by an unbelievable 137 tsunamis

in the past 200 years, from all directions.

These are rare images of a tsunami that hit Hawaii in 1946.

It was caused by an earthquake in Alaska, 2,500 miles away.

Within five hours,

it had sped across the Pacific Ocean and hit Hawaii,

killing 159 people.

Within 12 years, Hawaii was struck three more times.

In 1960, the largest earthquake ever recorded

occurred off the coast of Chile.

It generated a tsunami that headed straight across the Pacific towards Hawaii.

Within 15 hours of the earthquake,

this tsunami crashed into the Hawaiian coastline,

killing 61 people.

These events happen so frequently, it's no wonder that Hawaii is called

one of the tsunami capitals of the world.

But Hawaii has one advantage.

Because of its location, right in the middle of the Pacific,

it takes tsunamis several hours to reach it.

Which means that Hawaii has usually got enough time to issue a warning and clear the coastal areas.

But that's not the case for Japan which, for one key reason,

often only gets a few minutes' warning.

And this is because Japan sits almost on some of the most

active faults lines in the Pacific Ring of Fire.

These faults are so close to Japan that if an earthquake happens here,

it can create a tsunami that will hit Japan within minutes.

This means those living near to the coast are given little warning,

and often the wave arrives before they have time to run away.

And this is why the Japanese death toll from tsunamis is one of the world's highest.

Tragically, tens of thousands of people have lost their lives

to tsunamis since records began.

It's no wonder the Japanese people

are now perhaps the most mindful of the power of the ocean.

Having been regularly bombarded by tsunamis,

Hawaii and Japan have become the centre of advanced warning systems,

systems that have already saved many lives.

Within minutes of an earthquake anywhere in the Pacific,

scientists can assess the size of the quake,

calculate the risk of a tsunami and, if necessary, issue a warning.

But even with a warning system, both these island communities

will always be in the path of every tsunami that has crossed the Pacific.

In the end, these islands will always be in the wrong place,

at the wrong time.

And now for a real rarity.

A tsunami that happened in a place where they never normally occur.

It happened in Britain.

It was one of Britain's worst natural disasters and at the time,

it was a media sensation.

In fact, the only reason we know so much about it,

is because it was the first tabloid coverage of a tsunami.

But it wasn't in the 20th century, or even in the 19th -

it was 400 years ago.

Who would have thought that we in Britain

may have had our own tsunami?

In fact, they're so rare here that we've only ever had three in recorded history.

And this is because Britain sits in a fairly safe geological area.

But in 1607, we were hit by what scientists believe

was either a tsunami or a massive storm surge.

It caused a media sensation,

and this was how the media of the time covered it.

These pamphlets reveal the full horror of the wave

that struck the counties of Somerset, Gwent and Monmouthshire,

along the Bristol Channel on January 20th, 1607.

But what on earth caused this rare disaster off our shores?

Some scientists believe a fault line that's rarely active

shifted off the coast of Ireland.

It displaced enough water to generate a tsunami.

Moving at speed, the wave would have started to head

in the direction of the Bristol Channel.

We know from the pamphlets that it happened at dawn.

People were just waking and starting their days.

At nine o'clock, their lives were thrown into chaos.

A huge wall of water hit the shores

and surged inland towards their villages.

Eyewitness accounts in the pamphlets report that the wave travelled faster

than a greyhound can run.

It was a time when very few people knew how to swim and many drowned.

The survivors clung to the steeples of churches

and to the roofs of the few buildings that were left standing.

It swamped entire villages, people and livestock.

It had swept inland for up to 200 square miles

and took the lives of 2,000 people.

News of the disaster travelled fast.

And just like today,

the first to arrive at the scene were the media...

hungry for a story.

Then, as now, disaster sells.

In these cruel waters, many men, women and children lost their lives...

..an overflowing of water and forcible breaches made into the firm land...

The sudden terror whereof struck such an amazed fear into all the inhabitants...

Flocks of sheep are destroyed...

Everyone prepared himself ready to entertain the last period of his life...

Dead bodies float to the surface and are continually taken up...

The whole country shall feel the smart.

Today, we're now able to explain the causes of tsunamis with science.

But it's clear from these pamphlets what the media at the time

believed caused the event.

To their minds, God had sent this as an act of retribution.

For them, God sends weather and God sends waves.

Divine vengeance or not, these pamphlets have allowed us

to piece together the awful events of that morning.

And the 17th century media coverage

recorded one of the worst ever natural disasters to hit our shores.

My next story is one of the best kept wartime secrets from World War Two.

It reads like the plot line of a James Bond novel.

But I assure you it's not fiction.

This is a story of the development of a bomb.

But not just any old bomb - a bomb that would trigger a tsunami.

The war in the Pacific.

For several years during the Second World War,

much of the Pacific was a battleground,

with fierce fighting between the Allies and the Japanese,

not just on the mainland,

but also on many of the thousands of tiny islands

scattered across the Pacific Ocean.

The Japanese had captured many of these islands

and were so well entrenched, it felt like they couldn't be defeated.

The terrain was difficult and there were huge casualties.

So the Allies were keen to explore new ideas about how to fight,

without risking so many of their troops.

They wondered if there was a way of fighting the Japanese here which didn't involve soldiers at all.

It was in 1944 that the idea first began to dawn on allied generals

that there might be a simpler way to wipe out the enemy.

That it might be possible to create artificial tsunamis

by exploding bombs in the ocean.

The plan was that these man-made tsunamis could be set off

in the direction of the Japanese-occupied islands

and drown enemy soldiers.

Codenamed Project Seal, this plan was considered so significant at the time

that the Allies classified these documents as "top secret".

And they have only been recently revealed.

The mastermind of this project

was to be an Australian, Professor Thomas Leech.

And he became obsessed with building a tsunami bomb.

He was convinced he could create a wave 40 feet high

that would travel three miles inland,

swamping enemy-held beaches and drowning the enemy.

And so the experimenting began.

His team didn't just use explosives out at sea,

they also built a special pond as a laboratory to test their bomb.

It was thought that at a certain depth, underwater explosions

might generate surface waves much larger than usual.

So they planted underwater explosives and detonated them.

But on the first day, this idea was blown right out of the water.

No matter what the depth, the waves formed were negligible.

Professor Leech then came up with a new idea.

He decided the best way was to detonate explosives on a raft on the surface of the water.

EXPLOSION

And when they tried it, it did produce a few encouraging waves.

But as he started to increase the size of the explosions,

he found little increase in the size of the waves.

And from then on, as the experiments continued to fail,

Professor Leech's science and data also became suspect.

By the end of seven months,

he'd carried out an incredible 4,000 experiments.

But again, every single one of them failed.

Not one explosion did any more than agitate the upper levels of the pond.

Finally, our Professor Leech decided on the biggest experiment of all.

He planned to carry out a huge test with 2,000 tons of explosives

in an inlet with the exact same conditions as Tokyo Bay.

But this experiment never happened.

Top military scientific advisers were by now deeply concerned by the failure of his experiments.

They looked over Leech's calculations

and could see they were flawed.

Some of his crucial equations were wrong.

One adviser said the amount of explosives needed to create

a tsunami were so vast that it would be impractical and would never work.

Project Seal was closed down.

The failure of the project was kept a secret...

until now.

And that was the end of one of the most unlikely tsunami stories that I've ever come across.

Many of us are brought up on tales from the Bible.

And my next story is a biblical tale that we're all familiar with -

it's the story of Moses.

But what some people may not know is that it's been linked to a tsunami.

According to the Exodus story,

around 35,000 years ago, Moses led his people out of slavery in Egypt.

But as the Hebrews left Egypt,

Pharaoh changed his mind about letting them go.

So Pharaoh sent an army of 600 chariots in pursuit.

And here's the part we all remember in the Bible.

So as to save Moses and his followers,

God parted the Red Sea so they could all escape across dry land.

And then the waters returned.

The sea closed over Pharaoh's army and drowned them.

That's the Bible story we all know,

but there's a twist to the Moses story.

Some experts now believe that instead of crossing the Red Sea,

Moses and his followers actually crossed a marshy area

north of that once known as the Reed Sea.

It was thought to be just east of the Nile Delta

and most importantly, near the coast of the Mediterranean Sea.

If you read the Bible in the original Hebrew,

the word 'red' is mistranslated.

In the Hebrew bible, Moses and his people

cross the "Yam Suph" - this means the Sea of Reeds.

So, not the Red Sea, but the Reed Sea.

And not a narrow seaway, but a shallow, marshy area near the Mediterranean coast.

Now you would think that parting the Reed Sea and then flooding it again

could only be achieved through divine intervention.

But there is a way that nature can perform exactly the same thing.

And of course, it involves a tsunami.

And this is how nature can do it.

One of the largest of all volcanic eruptions took place 3,500 years ago,

around the time that some experts date the Moses story.

It was that massive volcanic eruption at Santorini,

the one that devastated the Minoan civilisation on Crete.

The eruption and collapse of Santorini would've been so violent,

it would've displaced a huge body of water, creating tsunamis.

But crucially, when the sea rushed in to fill the void,

it would've sucked water in from all the surrounding coastlines...

even from the Nile Delta.

It's the drawback effect that happens just before a tsunami hits.

As the sea was sucked away from coastal areas, billions of gallons

of water would have been siphoned off the marshy Reed Sea in Egypt,

drying it out for up to 20 minutes.

It would have created the same effect as God parting the waves.

Then suddenly, the tsunami itself would have arrived on the shore,

flooding several miles inland and gushing up the river valleys.

Creating exactly the same effect as the waters returning and drowning Pharaoh's army.

If the Santorini eruption really did produce this type of tsunami in Egypt,

it would have been remembered for generations,

and may well have been the inspiration for this great story.

But it's also an example of how the forces of nature

can achieve an effect so unbelievable

that it seems like an act of God.

My next tsunami is truly extraordinary

because it's the highest wave ever recorded,

higher than any skyscraper on Earth -

it was half a kilometre high.

What's even more incredible is that a father and son, out on their boat,

not only witnessed the wave,

they surfed it and lived to tell the tale.

It all happened in a peaceful-looking inlet

on the northeast shore of the Gulf of Alaska.

This is the scene of the highest wave ever recorded.

The cause of the wave turned out to be something that took scientists completely by surprise.

The wave reached an astonishing 520m,

higher than the tallest building on Earth.

And what's more, there were witnesses.

On a clear and calm summer evening in 1958,

Howard Ulrich and his son were on their boat inside this bay.

The date was...

July 9th, 1958.

We came into Lituya Bay about eight o'clock in the evening.

My son was with me.

I was eight years old at the time and being a child like I was,

I was halfway asleep as well.

At approximately 10.15pm,

there was a large rumbling noise from up at the head of the bay.

It was like a big loud noise from...

over in this direction towards the mountains over there.

There was a slight pause.

I thought that everything was over with...

but some movement caught my attention out of the corner of my eye

and so I looked directly up there and what I observed was a, er...

like an atomic explosion.

After this big flash came a huge wave.

It looked like just a big wall of water.

He threw me a life preserver and he said, "Son, start praying."

You're looking at death and this is exactly my first thought.

When the wave hit us, I did feel the boat all of a sudden

start shooting upwards, skywards.

I had 40 fathoms of anchor chain and it started running out off the boat.

Came to the end of the 40 fathoms, just snapped it like a string.

And then we were free, but we were still on the front of the wave.

We were swept up over the land and up above the trees.

That's where I assumed that we were going to end up.

But instead, they rode the front of the tsunami

as it carried them high above the trees for hundreds of metres,

before washing them back into the bay.

We still have the original 1958 conversation

between Howard Ulrich and the coastguard after it was all over.

I had never heard or seen of anything like this. It was unbelievable.

I couldn't imagine what could have caused anything.

I kept wondering just what mechanism could cause something like that.

A scientist flew over Lituya Bay the following day and filmed this footage of the devastation.

When he finally saw what had caused the wave,

it came as a complete surprise.

90 million tons of rock and ice had fallen in a massive landslide

into the head of the bay.

Amazingly, scientists had never before realised

landslides could cause a tsunami.

This cascade hit the water with a mighty impact

and released enough energy to create a wave surge so high and powerful,

it stripped trees and soil for up to half a kilometre up the slopes.

At this height, the wave would have swamped the tallest skyscraper on Earth.

50 times higher than a normal tsunami,

it was greater than any wave heard of in history.

This wall of water surged down the bay,

picking up Howard Ulrich's boat.

Surfing the front of the highest wave ever recorded,

father and son unbelievably got away with their lives.

The Lituya Bay tsunami taught us that massive landslides can cause tsunamis.

That might mean we can work out where big tsunamis might happen.

Which is why scientists are looking around the world's coastline

for places where there are unstable slopes

that might send giant landslides into the ocean.

And that's why I'm here on La Palma in the Canaries.

Because there's some evidence that that...

whole slope of the volcano behind me might one day collapse into the sea.

How it will happen is the subject of my final story.

The Canary Islands are a chain of volcanic islands

lying just off the coast of North Africa in the Atlantic Ocean.

One of the most active of these is the island of La Palma.

And it's this very landscape that some scientists say

will one day cause havoc and devastation across the Atlantic.

These scientists predict that this side of the island

will collapse into the ocean,

creating a tsunami of titanic proportions,

far bigger than anything ever witnessed before.

The island is made up of two volcanoes.

This one in the south is called the Cumbre Vieja,

and is not only still active, but very unstable.

We know this because of something that happened

after the last eruption of this volcano in 1949.

About a week or so after the eruption in 1949,

something extraordinary happened here.

There were several very strong earthquakes, and this crack began to open.

The west side of the volcano, over here, started to collapse and slide towards the sea,

which is something you don't normally see on a volcano.

What it means is that the volcano is unstable,

which is why some scientists believe that this whole side of the volcano

may one day, during an eruption, fall into the sea.

But scientists can't tell how many more times this volcano has to erupt

before its western flank finally collapses.

But if it does happen, it would be one of the worst natural disasters in human history.

A huge section of southern La Palma,

weighing half a trillion tons, would fall into the Atlantic.

The impact of this colossal landslide

would displace a vast body of water.

It would unleash much more than just a tsunami.

It would be a mega-tsunami -

a wave that would start out well over half a kilometre high.

The wave would then radiate out across the Atlantic,

racing towards North America,

reaching the east coast in just eight hours.

The La Palma mega-tsunami, this mighty wall of water,

would hit Boston, New York,

then all the way down the coast to Miami and the Caribbean.

If you were standing on a beach,

the very first effects you'd probably see is drawback.

The ocean would suddenly just pull away. But in the background,

you'd be looking at a wall of water that would keep coming towards you.

It would engulf the whole US east coast,

sweeping away everything in its path up to 14 miles inland.

Every city on the shoreline would be destroyed.

So when will this catastrophic event happen?

Nobody knows. Nobody knows even when the next eruption will happen,

and it'll probably take many more eruptions before this flank of the volcano finally collapses.

So scientists can't say when,

but some are convinced that one day it will really happen.

If this event were ever to occur,

it would create a tragedy on an unimaginable scale.

Just because we've never experienced such an event,

we believe it'll never happen to us.

But from the age of the dinosaurs to the present day,

the tsunami stories I've told you

have shown us that they've always happened.

So, there's one thing we can be sure of -

tsunamis will continue to strike us at any time...

..and from anywhere.

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