The Killer Wave of 1607 Timewatch

16th of August 2004.

A freak storm strikes the village of Boscastle in Cornwall.

Without warning, a wall of water tears through the village,

destroying houses and sweeping 80 cars into the sea.

The damage will run into millions.

Nearly 100 people are airlifted from their flooded homes,

their lives saved by 21st-century technology.

400 years earlier, the peoples of the Bristol Channel were less fortunate.

On the 20th January 1607, another freak wave swept across the lowlands of the south-west.

It too came without warning and left 2,000 dead in its wake.

Yet for centuries this apocalyptic flood has been forgotten

and only now are scientists piecing together the evidence left behind.

Was it just a huge storm?

Or was the killer wave of 1607 in fact a British tsunami?

It is the winter of 1607.

The Stuart dynasty is not yet four years old and Britain is at last a united kingdom under James I.

This year will see the premiere of Shakespeare's Anthony and Cleopatra.

In the new North American colonies, Pocahontas will save John Smith's life.

In London, the Thames will freeze over.

The most remarkable event, however, will be forgotten -

the greatest flood in Britain's history.

On Tuesday 20th of January, as dawn breaks over the villages and hamlets of Somerset, Gwent and Monmouthshire,

there is no sign of an impending tragedy.

In these backwaters of the Bristol Channel, life is dominated by the steady rhythms of agriculture.

Sheep and cattle farming are the lifeblood of the local economy.

The people are relatively prosperous, hardworking and God-fearing.

Around nine o'clock in the morning, this simple, ordered life will be thrown into chaos.

In Llanwern, Monmouthshire, four miles from the sea, the servants of Mistress Van prepare her lunch.

In Goldcliff, of Gwent, William Tapp, church warden, makes ready for morning service.

In Berrow, Somerset, a milkmaid heads for work.

In the same village, John Stoles,

father of five, wakes late, unaware he will not survive the day.

No-one has any warning.

In an age when few people knew how to swim,

any caught in the freezing waters will be lucky to survive.

One Mistress Van, a gentlewoman of good sort, her house being four miles from the sea,

having seen the approaching waters, was surprised by them and destroyed

even before she could get into the higher rooms of her house,

such was the speed of the waters.

There was a maid that went to milk her cows in the morning

but before she had fully ended her business, the vehemence of the waters increased

and so suddenly environed her, she could not escape thence

but was forced to make shift up to the top of a high bank to save herself.

John Stoles was thrown down by the water.

He himself, with three or four of his children, drowned.

His wife and one of her sons were found the next day and survived.

According to eyewitness accounts, the dead perished in a mountainous wall of water

and that after the wave came a torrent that swept across the fields,

creating an inland sea of over 200 square miles.

The waters have washed many onto the rocks of poverty and misery.

But so have they brought some profit,

for seafaring men, I might call them thieves, come daily now in boats

and get richly laden with goods which they find swimming in the waters.

Many dead persons are sadly found floating

and as yet cannot be known who they are.

When the waters receded ten days later, they left behind a scene of devastation.

2,000 dead.

Hundreds of thousands of sheep and cattle drowned.

The local economy destroyed.

Men that were rich in the morning when they rose from their beds

were made poor before noon of the same day.

To this day, 20th of January 1607 remains the largest and most destructive flood in British history.

But until now, a full explanation for the disaster has not been scientifically researched.

Just why so many lost their lives remained a mystery.

When I was young I remember seeing in some books in a library some woodcuts of the flood -

pictures of people stranded up on top of high trees, on top of roofs,

rabbits even clinging to the back of sheep as they were floating along.

Very dramatic scenes which I've shown in my lectures to students for, well, getting on for ten years,

as a good example of what a storm can do.

Simon Haslett is a professor of geography from Bath Spa University College.

He grew up with the folklore of the flood.

You can't really imagine what it must have been like, other than the human tragedy of it.

Quite catastrophic and how people actually dealt with that is amazing.

For most of his life, Simon has accepted the conventional explanation of 1607.

A lot of the commentators on the 1607 flood have put it down to a storm coming in

and as a child you just accept what you're being told by the scientists and the historians

and you don't really question it.

Attributing the flooding of 1607 to a storm makes sense.

The area is famous for them.

-NEWS REPORTER:

-The sea defences have been breached

at more than a dozen points along the coast

and there's now concern about tonight's high tide.

On December the 13th 1981,

the sea defences along the Somerset coast were breached by a storm-driven tidal surge

and the lowlands behind them were inundated, as they had been in 1607.

As the waves swept through seaside villages during the night,

they carried away cars and parts of houses.

Officials say it was a miracle no-one was killed or seriously injured.

The lowlands of the Bristol Channel have always been prone to flooding.

Much of the area is below high-tide mark and has been protected by sea walls for 600 years.

This weakness is exposed when heavy storms coincide with high tides.

It's exactly what happened in 1981.

Low pressure over the Irish Sea

drew a huge volume of water to the mouth of the Bristol Channel.

A combination of high tide and strong winds

then forced the swollen waters back against the Somerset coast.

What should have been just a high seasonal tide

became a storm surge.

As a local man, Simon is well aware of the dangers.

If you live on the Levels, you're always aware of the vulnerability.

Although it looks tranquil, it actually has a record of disaster

and if a big event comes in, a big flood comes in,

then it can actually tragically lead to a huge loss of life.

Fascinated by the scale of the 1607 disaster,

Simon decides to meet with witnesses of the 1981 flood -

the biggest in living memory.

Ken Burrell lives in the same house he did 23 years ago.

In those days there was a bathroom down there and that's about a foot lower than this room.

My daughter was going to take a bath and water was actually coming through the bath panel.

I thought it was a burst pipe, so we said forget about that.

Came in and maybe...

20 minutes, half an hour later, that's when I started looking out through here.

Coming dusk and the first thing I saw was a row of black things coming towards me.

And that was actually the leaves being picked up by the water,

not that fast, maybe a fast walking pace.

Stood here and saw the water deepening and then getting a little bit deeper

and then it started to come up to the window.

That's when I said to my wife and kids, "Time to get upstairs."

And what damage did it do to your property?

Apart from knocking furniture about,

it just brought in a slow, steady flood level.

A flood level I marked on the doorjamb the day after.

I carved that mark in the doorjamb which was the height of the water throughout this room.

Further up the coast, Simon meets Thelma Blake, a farmer at the front line of the storm.

Much of Thelma's land is below high-tide level and is only protected by the sea defences.

It just come cascading down the bank and on through, like.

I mean you just had to make sure all the cattle run was all right, you know.

-So just hoping it wasn't going to get any deeper.

-Right.

Thelma lost just six sheep in the flood that night.

1981 was the worst tidal surge flooding in 100 years.

Yet the church in Kingston Seymour reveals how little damage it actually did.

In the 1981 flood, the church here wasn't flooded. It was dry.

In 1607, the water was five feet high here in the church

and most of that water actually was here on the ground for about ten days afterwards.

The level of the 1607 flood is recorded in five other churches on both sides of the Bristol Channel -

all record flood levels that make 1981 pale in comparison.

1607 was a local disaster unlike any other before or since.

News of the catastrophe spread fast.

As the waters retreated, the media of the day arrived to report on the event.

The waters as they did come down on their first entry.

So much did happen. So much terrible devastation.

They recorded the graphic accounts of destruction and lives lost

that appear in six different pamphlets written and published at the time.

It is from these eyewitness accounts that the full horror of the 1607 flood unfolds.

Upon Tuesday, being the 20th of January last past,

there happened an overflowing of waters and forcible breaches made into the firm land,

the sudden terror whereof struck such an amazed fear into the hearts of all the inhabitants

that everyone prepared himself ready to entertain the last period of his life's destruction.

The pamphlets revel in the details of death and destruction.

Then, as now, disaster sells.

It may be the case that the pamphleteers exaggerate in order to profit,

although I don't think we really know enough about the 17th-century book trade

to be certain that the bigger the lie you told,

the more copies you would sell.

That might be an attitude we're importing from the 21st century

back into the 17th century.

The quantity of detail about local geography for example, suggests that this just isn't made up.

One pamphlet is clearly written by a local as a pamphlet.

He has written it to present a local testimony about the flood and the damage done.

This is a direct communication

from an author who was, if not an eyewitness, at least close to eyewitnesses.

These pamphlets are published upon occasion

and they're published because something sensational has happened.

Usually what we find reported is based upon fact.

Page by page, they set out a chilling roll call of the villages

struck by the wave and of the lives lost.

In Brean Down stood nine houses and of those seven were consumed

and with them 21 persons lost their lives.

All the counties along on both sides of the River Severn,

from Gloucester to Bristol, which is about some 20 miles, were all overflown.

In some places six miles over. In some places more, in some less.

26 parishes in Monmouthshire were inundated

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

There happened such an overflowing of waters into the boroughs of Monmouth, Glamorgan, Carmarthen

and diverse and sundry other places in South Wales.

In Bristol, all the houses standing upon the quay near the waterside were all overflown with water.

Everything lies melted and soaked in grime and salt water.

Taken in their entirety,

the pamphlets reveal an unparalleled chronicle of disaster

and the full extent of the flood.

Over 200 square miles of land lost to the sea.

With this information, scientists at the Proudman Oceanographic Laboratory in Liverpool

can model the precise storm conditions needed to produce the 1607 flood.

1607 is a fascinating event.

Although we've observed surges in the Bristol Channel before,

we've never seen one of that magnitude.

The data is excellent because it allows us to piece together the extent of the flooding

and also the depth of the flooding at many locations.

Critical to interpreting the 17th-century measurements is the height of the tide that day.

The morning of the event, we have a very big tide. It's almost eight metres above ordnance datum.

That's one of the biggest tides you can get in the Bristol Channel.

So we know that there was a massive tide on that particular morning.

Kevin can transform this tide into a storm surge by adding hurricane winds of 80 miles an hour.

Only then do the flood waters of 1607 become a computer reality.

The warm colours, the reds and the oranges,

represent a metre and a half to two metres of extra water due to the surge.

You can see how that amplifies into the Bristol Channel.

Ten metres of water above normal sea level. Two billion tons of water were probably involved in the flood.

Was it this cruel coincidence of high tide and hurricane winds that made 1607 the most deadly of storm surges?

It seems to be proof positive of the assumptions that for years Simon Haslett took as fact.

But in 2002, Simon made his own discovery

and it has forced him to consider a more shocking explanation for the 1607 flood.

Today, Simon is meeting again with the Australian geologist Ted Bryant, with whom he made that discovery.

-Hey, hi, Ted. How are you?

-Not bad.

-Easy journey?

-Yeah, oh, till we got to Bangkok.

Yeah? Oh.

Over the next two weeks they intend to collect evidence from around the Bristol Channel

that will substantiate their revolutionary theory -

a theory sparked by a chance discovery in a country church.

There it is, what we saw two years ago, "The great flood, AD1606".

Yeah, that's still as impressive as the first time I saw it.

The date reads 1606 because at the time, the new year did not begin until March.

Above the inscription, over five foot off the ground,

is a mark showing the level of the floodwaters.

But it was what they found inside that really stunned them.

As they thumbed through a history of the church,

they came across an extract from one eyewitness account recorded in the pamphlets.

They describe these waves as mountainous and the line was,

"Such a smoke as if mountains were on fire

"and to the view of some it seemed as if myriads of thousands of arrows

"had been shot forth all at one time."

So there's obviously sparks coming off this wave.

After we read this, we looked at each other and said, "That's not a storm."

That's a description of a tsunami.

Suddenly, the pamphlets had new significance.

They may be the only eyewitness accounts

of one of the world's most destructive natural phenomena striking Britain -

a tsunami.

It would explain why the victims of the killer wave of 1607 never stood a chance.

The terrifying reality of a tsunami stunned the whole world on Boxing Day 2004.

The Asian tsunami, known to have killed 300,000, was triggered by a submarine earthquake.

Giant underwater landslides and collapsing volcanoes can also unleash similar disasters.

For the last 15 years,

Professor Ted Bryant has been defining the unique character of tsunamis across the globe.

A tsunami surges over the land,

so there's an enormous volume of water brought onto the land

and you don't see that under storm waves.

Even thousands of miles away from its source, a tsunami can have terrifying destructive power.

In 1960, a massive earthquake off Chile generated a tsunami ten metres high.

It travelled across the Pacific Ocean at the speed of a Boeing 707.

600km an hour.

And when it got to the other side of the ocean - Japan - that's half a hemisphere away,

it had enough force to wreck buildings, to drive ships onto the shore

and there was massive damage right around the whole rim of the Pacific Ocean in 1960

because of that earthquake-generated tsunami back on the coastline of Chile.

The eyewitnesses of some tsunamis have observed a strange and distinctive phenomenon.

They describe the crest of the waves as sparkling with strange lights.

The last such account was from survivors of the Papua New Guinea tsunami in 1998.

In Papua New Guinea, the tsunami came at twilight,

and again there were reports of flames. There were sparks coming off the top of the wave.

For Ted, the echoes of 1607 are uncanny.

The Redwick church one is about nine in the morning, it would have been daylight

and still there's this description of sparks.

We don't know what causes the sparks

but it is a characteristic of tsunami waves

and for Redwick people to see sparks on the top of the wave in broad daylight,

they were looking at some incredible phenomena coming towards them.

To Ted Bryant, the pamphlets are clearly describing a tsunami.

The size, speed and strange sparkling of the wave all fit.

It seemed as if millions of thousands of arrows had been shot forth all at once...

Those who saw the mighty torrent approaching say that the waters afar off

looked to be many yards above the earth and with such smoke as if all the mountains were on fire.

The like have never, ever been seen or heard of in the memory of man.

The pamphlets provide a foundation case for a tsunami

but Simon and Ted need physical proof to back up these chilling voices from the past.

They set out to scour the coast of the Bristol Channel for evidence.

At Dunraven Bay in South Wales, hundreds of boulders lie at the foot of the cliffs.

Some have obviously just dropped off the face but others are less easy to explain.

Which ones have we done? We've done that one over there, that one over there.

Have we done that one? All right?

To the untrained eye, all boulders look the same

but to Simon, each rock gives up clues to the events of the past.

This particular boulder has, I'm pretty sure, been moved off the beach.

It's got some fossils in it, which you don't normally associate with the older limestones

which you find on the cliffs here,

so it looks like this quite big boulder has come from over there on the beach.

The force of water needed to move seven-ton boulders could easily be produced by a tsunami.

The way the boulders are lying gives Simon and Ted another clue.

OK, that's 270 degrees west.

We're finding a lot of these boulders are actually sloping back

because they come to rest in an orientation

that offers least resistance to the flow going over the top of them

and we've got a lot of these boulders over here

which all point back in the same or similar direction.

A storm operates in splashes.

You've got a wave breaking and storms can move the odd boulder

and can fling boulders up onto the top of cliffs

but given that we've got so many boulders in a train - what we call a boulder train -

and they're all pointing back in the same direction. That suggests to us a constant flow over time.

By measuring the size and shape of the boulders,

Ted can estimate what height of tsunami would have been required to move them.

The equation we have is one that says, "This boulder is sitting at the edge of the beach.

"About water level.

"What force is required to lift it up and move it?"

And we have equations that relate that to the depth of the water and the height of the tsunami.

Ted reckons it would only have taken a five-metre tsunami wave to shift these boulders.

For a storm to do the same thing, they calculate it would have taken a wave at least 20 metres high.

Over 60 feet.

Yet the very idea of a tsunami laying waste to the Bristol Channel

goes against every assumption we have about Britain being geologically safe.

The widely held view is that storms batter us all the time but tsunamis never come anywhere near Britain.

But in fact, they do.

7,000 years ago, the entire east coast of Scotland was battered by a mega tsunami.

It was triggered by a gigantic landslide off Norway.

On an area of the continental shelf called Storegga,

billions of tons of sediment plunged from the shallows into the deep.

The scar of the landslide is still visible in sonar surveys

and from this evidence Norwegian scientists have calculated the size of the tsunami created by Storegga.

The wave that hit Scotland 7,000 years ago was 70 foot high.

Nor are British tsunamis confined to prehistory.

In 1755, an earthquake off the coast from Lisbon sent a series of tsunamis out into the Atlantic.

The south-west tip of Cornwall was hit by a three-metre wave.

If a third tsunami did hit the Bristol Channel in 1607,

the evidence should be extensive and not just on the shoreline.

Ted and Simon decide to investigate Rumney Wharf.

20 years ago, a survey of the marshes by a local archaeologist

revealed a strange anomaly in the sediment deposits.

Simon and Ted are hoping it might offer more evidence for a tsunami.

Well, the marshes here are very muddy and they've been like that for centuries

but back in the 1980s there was a survey done

that actually documented a sand layer within the mud deposits.

If the old survey is correct, the layer of sand should be visible on exposed sections of the marshes.

The same survey also proposed that the sand layer was left behind

by a massive surge of water from the sea around 400 years ago.

20 years on, is the sand layer still there?

And if so, what clues will it yield to Ted and Simon?

OK, Ted, I think I've got a dark layer here.

It's got...

It's coarse. It's quite sandy.

It's quite thin, here. It's coming to about ten centimetres.

Is there any pebbles in it at all? Can we see here?

No. No. No, but let's follow it round and see if it thickens.

Just around the corner, they find what they're looking for.

Gosh, it's got lots of pebbles in it.

And bits of shell.

This is heaps thick.

Not only is it sand but within it you have...

Well, just here small pebbles and also the little white flecks that you can see in here,

that's broken-up shell. Shell that's been smashed up and brought in here with the sand and been deposited.

Whatever force brought the sand here was an event of enormous power.

And for Ted, the sea shells rule out a storm.

Yeah, it's a good one.

It's a pipi, I think.

In these type of deposits you get them. You might be able to track it back to a source.

You look around you and there's no beach for miles,

so that's an indication that this stuff has been transported considerable distance.

The way some of the flow behaves, it will not abrade the material, so to find something very fragile

like this in this type of deposit is an indication that we're dealing with tsunami flow.

Microscopic analysis will provide more evidence of where the sand comes from

and how it got dumped onto the marshes.

To calculate the volume of sand deposited, Simon and Ted check how far the layer extends inland.

That's it. Right.

-Let's take this up really slow.

-Yeah.

OK, let's see what we've got.

Sand.

More sand.

And then clay.

So it was about 20cm thick here.

So it's tapering from whatever that was there, about 40, 45cm thick to about 20 here

and we peter out inland across the marshes.

For me, that layer - that layer of sand -

is such a stark difference to the rest of the estuary.

The waters of the estuary are full of mud.

The Severn is one of the muddiest estuaries in Europe and we have marshes.

That sand layer is out of place. It couldn't have got here unless we had this high-energy event.

If I came here and we couldn't find our sand layer than I'd have doubts that the tsunami didn't exist.

It was probably a storm surge.

But having seen those sand layers, the tsunami's sitting in the back of my mind well and truly.

At another site, 20 miles away, they find similar evidence.

Have a look at that.

And that's sitting right on that land surface.

We've got to explain how that got there.

-Well, you wouldn't get that just by floating in.

-No.

In total, Simon and Ted take samples from five separate locations around the Bristol Channel.

At all five sites, they find sand or gravel deposits and all in a single layer.

If the occurrence of sand layers in these marshes is due to storms,

you might expect to find more than one,

given that we do experience storms quite frequently.

But we have only got the one layer, which is interesting.

Back in his lab, Simon can examine the samples in more detail.

He is looking for microscopic evidence of where the sand originated.

Meanwhile, Ted heads out to Sully Island, a small outcrop of rock just off Cardiff.

It would have taken the full brunt of a tsunami moving up the Bristol Channel.

That's a major erosion.

Look at the big block over there that's collapsed in and the other rocks straight ahead.

Once again, big boulders seem to have been picked up and shoved against one another

by a massive movement of the water and on the headland, the top layer of rock has been eroded away -

exactly the sort of thing a tsunami could do.

So the tsunami will bash into this cliff, full force

and it could carve through the hardest rock.

It will carve through granites and salicified sandstones -

very resistant rock.

It just means nothing to it.

It just erodes them and it erodes it very quickly.

And this is as good as any evidence I've seen in New South Wales.

It's incredibly exciting.

However, dating the erosion on Sully Island back to 1607 is impossible.

Dramatic as this big-scale evidence is, it's far from conclusive.

But at a microscopic level, Simon has made a breakthrough.

These tiny spiral shells

are typical of the species that grow in the shallow waters inside the Bristol Channel.

But these shells are only found at much greater depths -

out in the open ocean, over 50 miles away from where they were deposited.

One of the sand layers that we're looking at here is from North Devon

and it's full of species of microfossils that have come from the Continental Shelf,

so this sand layer has been transported from out on the open ocean.

All of Simon and Ted's evidence -

the boulder movements, the sand deposits and the erosion of headlands -

reveals the 1607 flood in greater detail than ever.

Yet they have a problem.

None of it is unique to a tsunami.

At the Proudman Laboratory, the same evidence fits their explanation for 1607.

A storm surge is going to provide some billion tons of water

rushing across the flood plain

that's more than capable of picking up enormous rocks

and large amounts of sediment

and depositing them a long way from their origin

and as far as rocks and sediments are concerned,

they can't distinguish between one large, rushing volume of water and another.

And whilst the storm surge modelled at the Proudman Laboratory is of record proportions,

history and the pamphlets themselves do not rule out such a freak event.

The morning of January the 20th 1607 would indeed have been one of the highest tides on record.

Furthermore, three of the pamphlets begin their story of the flood by describing stormy weather.

In the month of January last past, upon a Tuesday, the sea being very tempestuously moved by the winds,

overflowed his ordinary banks and did drown 26 parishes...

And upon the highest of the spring, the wind blowing very hard at south-west,

there was such a flood of tide as the like was never seen in this town.

But not all of the pamphlets describe the weather of that day as being stormy.

One of the most detailed reports actually states it was a sunny morning.

About nine of the clock in the morning, the sun being most fairly and brightly spread,

the farmers overseeing their grounds and looking to their cattle perceived far off

huge and mighty hills of water tumbling one over the other.

Simon has been through the pamphlets time and time again

and believes the very brevity of their weather descriptions is significant.

In the pamphlets the weather only gets one or two lines,

so it seems to me that it wasn't of spectacular proportions.

There doesn't seem to be an overall impression of a huge storm,

one that would be necessary to actually cause the flooding that we have recorded.

Instead, the thing that really stands out for Simon is the detail with which they describe the wave.

There is an overall theme running through all the pamphlets

of a destructive event, very violent, disastrous, on a scale that is unprecedented.

The waters ran with a swiftness so incredible that no greyhound could have escaped by running before them.

Whole houses were removed from the ground where they stood

and were floating up and down like ships half sunk,

which came in such swiftness that the fowls of the air could scarcely fly so fast.

In contrast, observers of the 1981 storm surge

remember the flood waters advancing at only a fast walking pace.

It's a different character altogether.

Much more violent in 1607, with waters rushing inland at a velocity...

You know, some of the accounts say faster than a greyhound can run.

Nowhere is the comparison between the storm surge of 1981 and possible tsunami of 1607

starker than at the village of Uphill.

In 1981, the biggest storm floods of the last 100 years barely broke a window.

In 1607, the same village caught the full force of the wave,

as recorded in the fate of local landowner, John Good.

The gentleman with his wife and children got up to the highest room of the house.

There they sat comforting each other in their misery, hoping they might but go away with their lives.

Yet even that very desire for life put the gentleman

in mind to preserve something by which afterwards they might live

and that was a box of writing, wherein were certain bonds and all the evidence of his lands.

This box he got with much danger

and tied it with cords fast to a rafter, hoping there it would be safe.

But alas, in the midst of his gladness, the sea fell with such violence upon the house

that it bore away the whole building, rent it in the middle from top to bottom.

The gentleman in this whirlwind of waves got to a beam

and clinging to that was carried against his will for some three or four miles.

There he crept up and sat pouring out his tears

and to make him desperate in his sorrows

the tyrannous stream presented him with the tragedy of his dear wife and dearest children,

wrenched to their deaths by the torrent.

To the 17th-century mindset, such tragedy was evidence of nothing less than an apocalypse.

The readers of these pamphlets are not asking themselves,

"Was this a tidal wave? Was this the consequence of global warming?"

because the cause for the authors and for the readers is the same.

This is God. God has sent this. God sends weather. God sends waves.

So the root cause is the same and that's what's significant.

SHE SOBS

A BELL TOLLS

What is true for them is that this happened

and that this is a visitation by God. A warning of some kind.

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

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

Countless flocks of sheep are utterly destroyed.

The whole country shall feel the smart.

For Simon and Ted, the apocalyptic character of a tsunami

matches the testimony of 1607 far more convincingly than a storm surge.

But without a credible explanation for what triggered the tsunami in the first place,

Simon and Ted will struggle to persuade others.

They had assumed that their tsunami was triggered by a submarine landslide

but they are out of luck.

Detailed surveys of the continental shelf around Great Britain reveal no evidence of a landslide.

Their next best hope

is the possibility that an earthquake on its own could have triggered the tsunami.

At the British Geological Survey, Dr Roger Musson, head of seismic hazards, assesses that likelihood.

We're really driven to the conclusion

that it must have been an earthquake that was quite large

and produced a tsunami by actually breaking through the sea floor

and causing a vertical displacement.

The big surprise is that the sea bed off the south-west tip of Ireland

is far less stable than commonly imagined

and is the location of an ancient but massive fault line.

We still have this old weakness in the crust here

and it's been suggested that this is exactly the sort of place

where you could get an anomalously large earthquake happening.

It's not just an idle theory.

On the 8th of February 1980, sensors recorded an earthquake from exactly this area.

It was 4.5 on the Richter Scale -

not enough to lift the sea floor but violent enough to give fresh impetus to the tsunami theory.

So we know from geological grounds that this is a probable likely place

for getting an extra-large earthquake if we're going to get one anywhere around Britain

and we know from seismological evidence that we've actually had an earthquake here,

so there is a fault which is moving. It's active.

So putting a hypothetical large historical earthquake in this spot is not so fanciful.

To the oral history of the pamphlets, and the geological evidence they've discovered,

Simon and Ted can finally add a possible cause.

The final piece of their tsunami theory is in place.

This, now, is how they believe the killer wave may have struck 400 years ago.

On the morning of the 20th of January 1607,

an ancient fault line off the coast of Ireland shifted violently,

displacing enough water to generate a tsunami.

Moving at close to 100 miles per hour, the tsunami rushed up the Bristol Channel,

its force magnified by the high tide and the funnelling effect of the geography.

As it roared towards its unsuspecting victims,

it eroded headlands and pushed boulders aside like pebbles.

A wall of water up to ten metres high rushed over the low-lying sea defences either side of the Channel.

Now travelling at 30 miles an hour,

the killer wave bore down on the villages of Somerset and Monmouthshire.

In one giant slab of water, billions of gallons kept coming with terrible violence.

The people caught in its path had next to no warning.

And the force of the waters was such that even those who thought they were safe in their houses,

they were swept away also and the numbers and numbers of...

I'm doubly excited now about what we've found.

We've managed to go right round the estuary

and we've seen the physical evidence that supports the historical accounts.

There's nothing at odds there at all and everything is very consistent.

Whether it's sand on the salt marsh or it's pebbles in the clay

or it's erosion on the headlands or boulders piled up in key spots, you go for the simplest explanation

and I can put down most of the signatures we've seen in the past week very easily by one way,

one process, one point in time, and that's the simplest explanation.

I think it's a colossal event.

If it is a storm, it's a big one but if it's a tsunami,

it could be well within what we've experienced elsewhere in the world.

We're just not used to it here.

For the people struck down by the killer wave,

there is still no definitive answer as to why they died.

But their fate is not just a historical curiosity,

for what is not in doubt is the vulnerability of the Bristol Channel lowlands.

Where once there were only farms and hamlets are now modern towns and many thousands of people.

Tsunami or storm surge -

both could happen again.

Another freak storm would give us some warning.

A tsunami would not.

Subtitles by BBC Broadcast 2005

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