Original subtitles

Have you ever wondered why Britain is an island

and why these cliffs, the White Cliffs of Dover, look so very

remarkably similar to the ones over there on the other side in France?

Well, half a million years ago, they were joined by a land bridge.

It took a catastrophe, a one-in-a-million event,

to change all of that.

The land bridge that connected us to the continent was washed

away in the biggest flooding disaster ever seen in Europe...

..where a lake twice the size of Wales

drained in a matter of weeks,

gouging out the English Channel.

It was a megaflood and that flood changed the face of our nation.

As a palaeontologist and evolutionary biologist,

I'm fascinated by the awe-inspiring geological events that created

the island of Britain...

The earth must have shook.

Yeah, it would have been deafening.

I mean, a catastrophic event.

..the mysterious lives of our prehistoric ancestors...

That is beautiful.

..and the mega-beasts that lived alongside them.

Look at its enormous tusk.

I'm going to set out on foot to walk through time

and discover this long, lost world

before Britain became cut off from the Continent

in a one-in-a-million catastrophe.

My journey through time starts over half a million years ago

along the beaches of North Norfolk...

..where mega-beasts and ancient humans could walk across

the land bridge that connected Britain to Europe.

My journey continues down to Kent,

along the North Downs to the White Cliffs of Dover.

Finally, I'll go under water off the coast of south-east Britain

to witness the evidence for the megaflood.

Oh! Wow!

Great visibility. Yeah.

That's cool.

One million years ago these cliffs didn't exist.

They would just have been a rolling ridge

stretching as far as the eye can see into the horizon.

There would have been no Straits of Dover,

there would have been no English Channel.

But something happened to destroy that land ridge

and ultimately turn us into an island.

First I want to get an idea of what Britain looked like

when it was connected to the Continent. What lived here?

What animals? Who lived here?

Were there humans?

The best place to do that is the coast of Norfolk.

Norfolk was right next to the span of land that once

connected us to Europe.

My first impulse in any new place is to get

the lie of the land from a high vantage point.

Being seven months pregnant should never get in the way

of a good view.

I've just climbed 133 steps to the top of the tower

of St Mary's Church in Happisburgh to get my first

proper glimpse of the Norfolk coastline,

and what a view.

If I was here 800,000 years ago or so,

I wouldn't be looking at the sea at all,

I'd be looking across a really wide river estuary.

And so all around me I'd be looking down on lush river delta.

I might even catch the distant trumpet of a mammoth,

or the trampling of hooves of a large herd of bison.

In warm periods, this was a world populated by huge mammals

that came and went from the Continent across the land bridge.

If I was very lucky,

I might catch a glimpse of some people

wandering along the river bank.

Evidence for these ancient species of human

that were so very different to us today

is found here on the beaches of Norfolk.

And sometimes, so too are the remains of very, very big animals.

Alongside me is a deposit that is known as the Cromer Forest Bed.

In particular this bit is known as the West Runton Fresh Water Bed

because that's what it is.

We're on a beach but this here, all of this stuff,

is not beach sediment.

As you peel back the surface, you can

immediately see it's not sand,

it's soil.

It's the kind of thing that would be deposited in a really

slow-moving, tranquil river.

I kind of imagine slow waters,

marshes, some grassland around the edge

and in the distance maybe a forest.

A few stoats might run by, a mole, squirrel -

very quintessentially English, until a hyena sticks its head out

and is followed by a sabre-tooth cat or a lion, or a lynx.

Suddenly, you're in a slightly different world altogether.

That's West Runton 700,000 years ago.

Here on the beach at West Runton, something amazing was

discovered that had been hidden for hundreds of thousands of years.

Here we go.

In 1990, children's nursery assistant Margaret Hems

and her late husband, Harold,

were searching for fossils in the run-up to Christmas.

So, Margaret, tell me

a little bit about how you came down to the beach in the first place.

There was an announcement saying that there was going to be

a high tide, high winds and possible flooding at Blakeney

and my husband and I, we looked at one another and said,

he said to me, "An early start in the morning." That's what we did.

So you were walking along the beach and something caught your eye.

I saw this round piece in the bed and I knew it was a fossil.

Do have any photos? Yes, I have.

Look at that!

That one seems to be popular.

That is what you found? Yes.

Explain to me the bit that you saw first. This piece here.

I started digging carefully around and in the end it was the pelvis.

Wow!

My husband, when he saw that, said it could only be an elephant

but we didn't know what kind of elephant.

Margaret and Harold had made an astonishing discovery -

an almost-complete skeleton of the largest mammoth known to science.

How does it feel to have been the discoverer of one of the most

complete mammoth skeletons ever found?

It is very, very exciting.

It put West Runton on the map, didn't it?

It's internationally famous.

I remember thinking while we were digging it out,

I ought to be making my mince pies, not doing this!

THEY CHUCKLE

Margaret's mammoth is still undergoing conservation

over 20 years after its discovery.

This remarkable beast in front of me

is the head and tusks of the West Runton Mammoth.

Look at its enormous tusk, it's beautiful.

It's in absolutely exquisite condition and just this

portion of that mammoth skeleton can tell us so much.

From the size of its tusks we can guess it's probably a male.

It could have been over four metres tall,

probably weighed ten tonnes or more.

That's twice the size of a male African elephant.

Looking at its teeth, I can tell that he was probably in his 40s

when he died.

Now, that's an animal that should have been in his prime.

It was a steppe mammoth, the ancestor of the woolly mammoth,

which also frequented the British Isles.

Mammoths like these certainly didn't evolve in Britain.

Originating from Siberia,

they must have migrated here from the continent via the land bridge.

In total, over 200 bone fragments from the West Runton Mammoth

have been carefully catalogued and preserved.

There are so many bones and they're all beautiful

and they're all enormous. Look at this one here.

The femur, the thigh bone. This is about 1.5 metres in length.

An animal that is over four metres tall

is going to have pretty long legs.

Now, what's really interesting about this particular bone

is that it tells us that this poor thing had a gammy knee.

This side here is lovely and smooth.

That's what you'd expect where it would join up with the shin

to create the knee socket.

This side should be smooth as well but it's not.

Maybe a fall, maybe a fight,

but something quite dramatic caused this knee to pop out

and jump so that it only articulated on half of the shin bone.

So one whole side of the thigh bone wasn't in its socket.

It still survived despite this injury,

in a landscape that had lions, sabre-tooth cats

and all kinds of predators.

It was still OK but it wasn't really in

tiptop condition and that may have been what did for it in the end.

Now, this is fossilised hyena poo,

and the West Runton Mammoth was surrounded by them.

We know that either in life,

or hopefully for this creature maybe after it had already passed on,

hyenas got their fill and they ate an awful lot of it

and they left an awful lot of that meal behind

in the form of fossilised poo.

Britain was clearly a home to mega-beasts who had migrated

here from the continent.

Alongside animals like the mammoths, humans arrived to live

in a Britain which could be surprisingly comfortable.

In my day job I spend a lot of time imagining what the past

would look like.

Here on the Norfolk Broads, the landscape helps you out quite a bit.

Just looking about right now, I can really get a sense

of what the world would have looked like around here

a million years ago.

During warm periods, this part of Norfolk

would've been a water world much like this.

If you were canoeing here a million years or so ago,

you wouldn't be on the Norfolk Broads,

you would be on the ancestral River Thames,

which flowed all the way from Wales, right the way up to the sea.

The ancestral Thames was the original route of the great river

that now runs through London, hundreds of kilometres to the south.

This river would have run through heathland, heather,

there might have been some pine and spruce forests in the distance.

Sturgeon would have been swimming on their way to the sea.

The first humans who inhabited Britain are elusive

but astonishing clues to their presence are found in coastal towns

like Happisburgh.

Happisburgh is one of the most dynamic

sections of the Norfolk coast.

This here used to be the base of the steps

that run up to the top of the cliffs.

Except...

those cliffs are now all the way over there.

They've been retreating over the decades as the sea

has bitten away at them.

That same force of the ocean a few years ago, May 2013,

scoured away a good portion of the beach, the foreshore

and uncovered something quite remarkable

about a metre beneath my feet.

A huge storm had scoured the sand off the beach

revealing the mud layer beneath.

Over 50 mysterious indentations in the mud were discovered.

Human footprints.

Their age is truly awe-inspiring...

..around 900,000 years old.

With only a few hours to record them

before the next tide wiped them away,

researchers at the site took 3-D photographs.

So those footprints are now totally buried? Mmm.

No-one's able to see them any more.

Well, more than buried, they don't exist any more.

Washed away? They've been washed away.

David Waterhouse of Norfolk Museums has brought a 3-D print of one

of the ancient visitors' footprints to the beach where they were found.

This is the largest one.

This is a size nine.

Probably a male.

These adults were walking due south,

pretty much down the present-day beach but the smaller footprints

were winding all over the place and that just happens today, doesn't it?

Exactly... Parents walk along... Kids running around the outside.

Looking here at the footprints, the heel is back here.

There you can see the imprint of what looks like a big toe

and the other toes just in here.

It's just rather wonderful to think that this actually was a person

who was walking on a river bank, here,

maybe as long ago as one million years ago,

but at least 800,000 years ago.

They're the oldest outside of Africa, aren't they? They are.

The oldest footprints outside of Africa. Yeah.

You've got that massive leap from Africa to Norfolk.

That's why, you know, it's so special.

This amazing snapshot of human life was a one-in-a-million find.

The footprints were probably left by the mysterious Homo antecessor,

one of the earliest known species of human in Europe.

With a taste for simple stone tools and cannibalism,

so far their remains have only been found in Spain.

They would have migrated here across the land bridge.

Norfolk's rich fossil record tells us about a world that stretched

from half a million years ago back to a million years ago or so

where humans, mammoths, sabre-tooth cats,

a whole diversity of creatures could come and go freely

between Britain and Continental Europe.

That connection is underlined again and again and again

but it was about to change.

Humans and animals would come and go as the climate warmed

and cooled but an extreme cold snap would soon trigger

the sequence of events that would end in the megaflood,

Setting Britain on its way to becoming an island.

Half a million years ago,

Britain was connected to the Continent by a huge land bridge

that stretched all the way from Dover into northern France.

A geological catastrophe would change all of that

and leave the white cliffs of Dover plunging into what would

become the English Channel.

I am in Norfolk, looking for evidence of the people who crossed

the land bridge that once connected us to Europe so long ago.

Those haunting million-year-old footprints are not the only

type of evidence for ancient humanity in the British Isles.

Prehistoric Britons also left behind traces of their presence

in a unique type of stone.

The people of Norfolk have always made use of this natural

product of their environment.

And, like any good medieval building on the east coast of England,

this church is covered in the stuff.

St Mary's, Happisburgh, is a great example of medieval architecture.

14th-century church, it's made almost entirely of this stuff -

flint. It was all collected locally

and brought up here by the barrel load.

I mean, just look at that.

Imagine how many flint nodules would have been needed to

build this church. And that's how it came.

Like this, in nodule form.

In certain places, where they wanted to push the boat out a bit,

they would expertly split it in two

and place it, to reveal the beautiful lustre of its interior.

And these are all flush with each other, so it's called flushwork.

This is a great example of medieval craftwork.

But here in Happisburgh, there's even finer traditional flintwork

and it goes back half a million years.

15 years ago, during an especially low tide,

an extraordinary discovery was made here on Happisburgh Beach.

It was just this section in the sand

and I could see this straight line, which is...

It's just relatively unusual.

'Mike Chambers was the man in the right place at the right time.'

Out there, when the tide is out, the furthest you can get,

that's where I found it in the clay, that type of clay.

So, not in the sand. Not in the sand, no.

It was just this little edge. So, it came to Cromer Museum first.

Yeah, and it's people like Mike who know to bring these things in.

And, crucially, he remembered where it was from.

You can't date the actual flint, but you can date the sediment around it.

Shall we get it out? Yeah, I think so.

It's time! Brilliant.

'And here is Mike's discovery -

'at half a million years old,

'the earliest flint handaxe known in North Western Europe.'

Oh, look at that!

That is beautiful. It's so black and glossy.

Do we know how they used these tools?

This was really a butchery tool.

You would use these really thin edges maybe for skinning.

You could turn it round and use the thicker bits there

for maybe jointing. Do we know who made this?

From the shape of it, the design of it, essentially,

this is a species called Homo heidelbergensis.

Homo heidelbergensis lived between 600 and 300,000 years ago,

migrating in and out of Britain as the weather warmed and cooled.

With more sophisticated tools than antecessor,

heidelbergensis might even have been

among the first humans to bury their dead.

This handaxe was found out there, in that direction,

and half a million years ago, out there wasn't sea. No.

No, I mean, this was a riverbed we're talking about, isn't it?

We are in a riverbed and if you kept on going,

the heidelbergensis could walk all the way to join his mates,

because there was a land bridge between here and Europe

and this species was everywhere.

But here in Happisburgh, we have never found any bones,

any fossils of those people. Yet. Yet!

The humans who created priceless objects like this would not

stick around for ever.

The glacier that would eventually cause the mega flood

was moving across Europe, forcing them to migrate

out of Britain for sunnier climes.

A new ice age was on its way.

Ice ages are periods of intense global cooling where enormous

ice sheets descend upon the land, some kilometres thick.

Now, 450,000 years ago, there was a really big one

and the evidence for that is right here on this beach.

When we come down on the coast section here, we find that

the cliffs are almost entirely made of glacial sediment.

Geologist Martin Warren has been studying these beaches for decades.

To his trained eye,

the evidence for the Anglian glaciation that once covered

Britain in huge slabs of ice is written in the cliffs on this beach.

Here at East Runton we have got this amazing chalk that has been thrusted

and shoved, to make a hill.

So, there is chalk right up at the top of the cliff there that

really should be down at beach level

and it has been pushed around by the power of that ice sheet.

Wow, and you've got a 1km thick ice sheet that is advancing

and sometimes retreating and the power of that sheet

is moving these incredibly large blocks.

I mean, I can see what looks like some flint up there,

is that correct?

Yes, there is a great sheet of chalk, with lines of flint

running right the way through it.

With a trained eye like Martin at your side,

it is hard not to indulge in a spot of geological foraging.

This looks unusual. Yes, I think this is a piece of dolerite.

The most likely source of that, I would think,

is off the coast of Northumbria.

As the ice came streaming down the east coast,

it would have been bringing these down.

What is that, 100km, 200km, more?

240, I think, the last time... if I remember rightly.

How cool. Yeah. Was it rolled? Was it swept along?

The ice actually incorporates material

into the base of the ice sheet.

When eventually the ice melts, it dumps it here in Norfolk.

That's it, isn't it?

We often think of things like glaciers as being a bit static.

You worry about them melting,

but the whole time they are moving, moving, moving.

They are, they are very dynamic and very dirty.

Erratics, like the dolerite, are rocks carried

to the beach from far away by an advancing glacier.

They are a telltale clue that the ice sheet extended

far beyond this area.

Is that something here? Oh, wow, look at that.

That is an erratic. A bit of sandstone.

It has got a flat face on it that has been smoothed off

by the ice sheet and it has even got grooves and scratches

as it has been scraping along the bottom of the ice sheet.

That is an absolutely perfect example of an erratic.

Yeah, moved by an ice sheet 450,000 years ago. Ha!

Britain was being crushed under the advance of the most severe

glaciation of the past million years.

This deep cold snap forced humans and animals to flee this land

and it would set in train a sequence of events that would

lead to the catastrophic mega flood that made Britain.

So, 450,000 years ago, Norfolk would have looked

considerably different than today.

On this spot, there would have been a huge ice sheet

some kilometres above me.

This glacier would not only have covered Britain

all the way down to London.

Over thousands of years, a massive lake also began to develop.

That glacier was dumping all of its meltwater into an enormous

proglacial lake, and I really mean enormous -

we are talking twice the size of Wales.

It would have stretched all the way from here to the Netherlands

and down the coast some 200km, where it was dammed

by the enormous chalk ridge.

That ridge was down south, where the chalk land bridge

once connected us to Europe.

As I travel south, it's like I am being pushed by that glacier

right the way from Norfolk all the way to London and beyond,

because of course, that glacier was the beginning

of something much bigger.

Something was brewing and it was about to break

and that happened right down south, in Kent.

The remains of that ancient chalk ridge are here on the North Downs,

just outside Maidstone in Kent.

I'm actually currently walking along a ridge,

and there is a thin layer of soil, but beneath that, it is all chalk.

Now, this ridge that I'm standing on stretches another 40 miles or so

to Dover and, of course, back then, it stretched even further

and continued all the way into France.

The massive glacial lake built up between Britain and northern Europe.

It was the chalk land bridge that connected us

to Europe that held those waters back.

And that ridge was effectively acting as a dam.

It was holding back the waters of that massive glacial lake.

Flashes of white in the landscape reveal the deep chalk beds

under my feet that make up the ridge.

This is a unique habitat.

Suddenly, I catch sight of a creature that only

makes its home on this chalky downland.

One of the things I really wanted to see

when I came up here on the chalk ridge was a chalkhill blue.

He is hunkering down, to avoid this rather strong breeze.

And you get these blue butterflies on these chalk ridges

because their larvae feed on a type of plant that only really

grows in these chalky ridges, the horseshoe vetch.

He is just exquisite.

Today, the ridge is not just a beauty spot

in a long-forgotten link to the Continent,

there is another startling piece of evidence that this is the same

geological feature that appears in northern France.

And it is evidence that you can taste.

This corner of south-east England is a great place to

exploit our deep connection with the Continent.

It is all thanks to the stuff we are driving over - the chalk bedrock.

Here in Kent, a British wine grower is taking advantage of this

wonderful chalk ridge to grow grapes, to be made into a very

British version of an iconic French product - Champagne.

Chapel Down vineyard are cultivating grapes destined for

their award-winning wines, across 94 acres of chalky English fields.

The main feature here on the North Downs is this chalky soil.

It promotes a much warmer soil earlier in the spring,

which means these vines can come out of dormancy much earlier

than some vines which might be planted on

the colder, clayer soils of The Weald.

But the added advantage of chalk is that it acts

a bit like a sponge, so it will absorb water

and that then releases the moisture back to the roots gradually.

So, am I looking at, basically, the same sort of grapes you use

as you would see going into a bottle of classic Champagne? Yes.

I mean, here we are growing Pinot noir grapes.

Pinot noir is one of the main varieties

which they grow in Champagne.

These aren't into the ripening phase yet,

but they are developing very quickly.

EU regulations prohibit Chapel Down from calling their sparkling wine

Champagne, but that hasn't stopped them

from beating the French producers in several competitions.

Are people jealous? Well, I think they are.

They wouldn't say as much, but we have had the visitors at the

vineyard and they have been pretty impressed with what they have seen.

I'm imagining, you know, Champagne vineyard owners

with their binoculars on the ridge, eyeing up the land.

Who knows? You joke,

I have heard rumours of the champenoise coming over,

looking at our land and thinking it might be the next best thing.

Really? But we'll see. Next best thing?

Well, to what they think.

You're like, "We know it's the best!" Absolutely.

450,000 years ago, this chalk ridge that held back the waters

of the glacial lake was on the brink of destruction.

It was about to give way in the most catastrophic flood

Europe had ever seen.

I'm investigating a mega flood that happened half a million years ago.

A catastrophe that helped separate Britain from the continent

and turn us into an island.

I'm now at the south-east of Britain in Dover

where I'm seeing the cliffs as they are meant to be seen -

from the sea.

The White Cliffs of Dover, they are iconic and rightly so.

They are gorgeous, especially on a day like today

with that pure calcite glinting in the sunlight.

Bright white chalk in places 100 metres high, these cliffs.

But of course, that chalk extends further down, deeper

and from here in Dover, it extends across south-east England,

all the way into Norfolk.

But of course, it's their emotional link to the British Isles

that we really recognise them for.

The last thing that many a sailor is seeing

when leaving home for distant lands.

The first things that many a war veteran has seen when coming home.

They are the quintessential image, if you like, of our island nation.

In many ways, in many of our hearts, they define Britain.

At their highest point, the cliffs stand proud of the sea at 110 metres.

Taller than Big Ben

and the same height as 25 London buses stacked on top of each other.

You get used to thinking of these cliffs as being here forever

but of course, they are dynamic in their own way.

The sea is eating away at them, day by day

and they are actually eroding at a rate of about a centimetre a year.

Sometimes, that erosion happens faster and huge chunks fall off

and you can see on the cliffside, where it's slumped.

The Straits of Dover have been Britain's first line of defence

against invasion for thousands of years.

But the cliffs also did their duty during World War II,

to protect us from the Nazi threat.

A string of heavy guns were installed

with their sight trained on the Channel.

The tunnels that provided protection and living space for the soldiers

that manned those guns are known as the Fan Bay Deep Shelter.

Newly opened, the tunnels give me the opportunity to examine

this ancient chalk, laid down around 90 million years ago.

Well, here is the chalk, that's for sure.

It is the same stuff the Channel Tunnel's going through and taking us

to Europe today. It is a great tunnelling material.

And look down here.

Really, really striking, solid layer.

The flint is running all the way along...

..deeper and deeper in.

You can see the signs of the handiwork of the people

who were actually living down here.

Look, this is a bit of graffiti.

1940.

Amazing to think that all this stuff is actually the remnants of

billions of tiny, tiny marine algae that sunk like rain or snow

to the bottom of the sea floor.

And amongst those tiny organisms, there was also

some bigger things. I mean, look at this.

Maybe some kind of mollusc, definitely a fossil.

So, it would have taken maybe a million years

just for 20-30 metres of this stuff to be deposited

and these cliffs are 300 metres high in places.

That is ten million years of accumulated animal life

built up there.

It is quite wonderful.

450,000 years ago, this massive layer of chalk would have

extended across the English Channel into France.

Dr Jenny Collier of Imperial College London

has long been fascinated by the disappearance of the land bridge.

It is actually really easy to understand that it actually

is the same rock unit going all the way across.

You can see the white cliffs over there in France and they continue

right across here to our Dover cliffs.

When the land bridge existed all those years ago,

this view would have been very different.

This would actually all be dry land, out front of us,

where we've got the sea here.

As the chalk dipped down to lower levels,

it would have just continued down into a dry valley,

maybe with a few rivers. Yeah.

Jenny and her team discovered clues to what removed the chalky

land bridge when they were doing a sonar survey of the seabed

under the English Channel.

It had long been postulated that something dramatic

had happened to remove this great big rock ridge

but no-one really had the evidence for it and we actually found it

genuinely by mistake, by doing some surveys down to the west

towards the Isle of Wight.

And we just found this huge, great big valley,

carved into the floor of the English Channel about ten miles wide.

A valley as deep as this could only have been created

by an extremely powerful flood event.

For Jenny and the team, all evidence pointed to the huge glacial lake

full to bursting to the north of the chalk land bridge.

Because we knew we had the lake, this was obviously a perfect source

of the volume of water we needed and then we've also got the rock ridge

giving us all the lumps of material to make the gouges.

And to gouge rock, needs a high amount of force

so we'd have had lots of ice on the lake as well,

those would have been thundering down as well.

The chalk ridge gave way in a cataclysmic one-off event.

Across a period thought to be as short as three weeks,

over a million cubic metres of water per second broke through,

destroying the ridge and crashing down onto the valley below.

I mean, today, the flow rate over Niagara Falls, for example,

it would be 300 times that.

That must have shook.

Yeah, it would have been deafening, a very dangerous place to be

and just a phenomenal, catastrophic event.

Would you have needed something else to fragment the ridge,

to sort of kick it off?

We know there are small earthquakes in this area

and perhaps there was a small earth tremor that just set it off.

A seismic event, this time a volcanic eruption, was the trigger

for a similar but much smaller glacial flood in Iceland in 2010.

This footage, shot from a news helicopter at the time,

is both fascinating and slightly horrifying.

It gives you an idea of just how ferocious

and apocalyptic these events can be.

Jenny and her team have only ever visualised the underwater channel

using sonar.

They have never set eyes on it.

So, we have come to Gosport to do something

that has never been done before.

This is going to be an interesting day.

It is pretty exciting. We are going out to roughly halfway

between Britain and France in the middle of the English Channel

and we're going to go deep underwater, 70 metres deep,

to see what is down there. Hopefully, we will find some

evidence of the mega flood that Jenny has seen on her sonar scans.

However,

the wind's picking up...

..we are in the middle of a shipping channel, we are diving deeper than

most people tend to dive in this area, it is completely unexplored

and we don't really know what is going to happen.

I'd like to say I was confident...

I am confident, no, I am completely confident we are going to do well.

Unfortunately, there is a lot working against us so we shall see.

But I'm not going to put on a wet suit.

Adrian Glover from the Natural History Museum

has brought along a submersible, remote-operated vessel,

nicknamed Rex, to act as our eyes underwater.

So it is a mini sort of remote control submarine

but with a tether, a line that connects it to your computers here?

That's right. We're way too deep for diving, where we're going.

That is what an ROV brings you, the ability to work beyond scuba depths.

We're going to see, hopefully,

evidence of the sides of these channels.

It is the freshness of the landscape which is so remarkable and it is,

I say, the tides have just swept it all out for us. Brilliant.

The trip to the edge of the underwater valley

will take a good three hours.

We've just left port and we are passing the Isle of Wight over there.

We are in the sheltered part of the Channel right now.

I can see the ships passing by out there, lots of container ships.

Yeah.

Onwards.

But enough worrying about getting run over by a tanker,

it is time to do some science.

What we're looking at on this map is actually a map of the sea floor

and it is these reds, these yellows,

these greens...are all telling you the depth and this blue,

that is the deepest bit yet and that is where we are headed for.

We are on our way out to the middle of the English Channel.

Here, we'll send Rex down to the seabed.

Through Rex's onboard camera,

We hope to gaze into the half a million-year-old mega flood valley itself.

'It's been a three-hour journey to the middle of the English Channel.

'Thankfully, the weather has improved.'

We're very close, within 100 metres or so.

Almost there.

'Finally, we're at the spot where we'll send our

'remote-operated submarine, Rex, underwater.'

So we're now, I guess, about here. Cool.

'Along with scientist Jenny Collier, I'm hoping to get a glimpse

'of the ancient channel carved by the mega flood that made Britain.'

So, are we almost here? Well, we are here. We're here?

So, what Graham's doing is just bringing the boat around,

so we're just doing a couple of profiles

just to make sure we're on the cliff edge.

'The skipper positions the boat over the valley

'that Jenny's team found on the seabed.

'We want to drop Rex on to the edge of the valley

'so he can get a good look down the slope of the mega-flood channel.'

Probably we're going to have Rex in the water in about 20 minutes' time,

but we want to make sure we've got no uncertainties,

it's exactly where the cliff edge is going to be.

It looks so featureless out there.

Expanse of water. Nice and calm.

But it does belie what's beneath us.

It's action stations. The team get to work, preparing for the dive.

OK, Adrian, we're all hooked up, ready to power up.

OK, vertical thrust, to check, coming up now. OK.

Main thrust is all good.

And lights check.

Lights OK. Happy.

With a clean bill of health...

Off she goes.

..Rex is over the side and on his way.

All yours, Adrian. Copy that, John.

OK, we're going down very fast.

We're now at 40 metres. Wow!

OK, still going down.

Well, it looks like a lot of snow,

but clearly it's not, cos it's underwater.

Going through a layer with quite a bit of

resuspended bottom sediment, I suspect.

'Worryingly, the water seems to be full of suspended sediment.'

OK, coming up on 45 metres.

4-5 metres.

'If the seabed is covered in mud and sand,

'we might not be able to see the drop off.'

We're at 56 metres now. I suspect any moment now we should touch down.

There's the sea floor. Oh, yeah.

Great visibility. Yeah. That's cool.

'At 60 metres down, there it is.'

We are basically on a slope on the seabed.

'Rex's position marker shows that he is sliding down into

'the vast flood channel discovered by Jenny and her team.

'Through Rex's camera,

'we are gazing at something never before seen by human eyes.'

It looks like a drop off.

We're seeing off the edge.

These are really prominent sides. It's like a precipice.

What you expected? It's better than I expected.

We're pushing it a bit at the boundaries.

There's a reason why people don't do this very often.

That's right. I'm quite nervous.

My heart rate is going up and I find this quite stressful,

and we're recording the data,

we're getting images of an area of the sea floor that,

basically, nobody ever looks at, because it's so hard to get to.

So, here's the edge of that 60-metre contour.

Down each of these contour lines, getting deeper and deeper and deeper

and, as you can see, Rex is this little red spot here.

He's being bounced down the edge of this slope.

'At first, the drop off looks barren,

'but marine biologist Adrian's eagle eyes

'spot a thriving community of sea life.'

Oh, that is a coral.

So, that is what's called a Dead Man's Finger,

Alcyonium Digitatum.

'As a suspension feeder,

'the Dead Man's Finger can thrive in the gloomy depths.'

You get these kinds of corals in UK waters.

This is an anemone. Oh, look, yeah. It's beautiful.

See that sea anemone on the bottom right?

There's actually really very high biodiversity here.

It's quite interesting.

If you just take a few square metres of this,

you'd find many, many species.

'All these species are adapted to fast-flowing currents.

'They feed on the nutrient-rich water

'flowing at high speed around them.

'And, if it wasn't for these rapid currents, the drop off might

'never have been discovered during the original sonar survey.'

Many people have said to us,

"It's 400,000 years old, why's it still there?"

And actually we think probably in the past,

it was covered with sediments, but today, because of these extreme

tides we have, it's all swept clean and we're seeing that today.

So it's being revealed to us for the first time in 420,000 years?

Absolutely, absolutely. It's remarkable.

It's great to see the mega flood channel with the naked eye.

There's a boulder. Small boulders there in front of us.

There's a bit of chalk.

'And it seems we have also found lots of chalk boulders.

'They could be debris from the chalk ridge

'that once connected us to France.'

A few lumps about this big, based on the size of Rex's pincers,

you do wonder, don't you, how far they've rolled

and when they first landed there?

The mega flood probably only raged for a few weeks but in the aftermath

of the catastrophe, the chalky ridge would have been obliterated.

In the hundreds of thousands of years to come,

Britain would have been accessible during periods of low sea level.

That huge chalk ridge that connected us to Europe

was a thing of the past.

We saw with our own eyes the evidence for this mega flood.

It really brings it home to you that the shape of the seafloor

that lies beneath, which you don't get at all any sense of when

you're out here, particularly on a calm day, definitely a privilege.

My journey has taken me from the first humans in Britain

to the mega flood that destroyed the chalk land bridge.

It's been a story of mammoths.

Priceless objects.

Amazing connections to the continent and fantastic scientific discoveries

that fundamentally changed the way we think about Britain.

But, after all this talk about us as an island nation,

I can't help feeling how connected we are to Europe.

We think of ourselves as an island nation, don't we?

But it took a catastrophic event of unimaginable scale to tear

us asunder from the Continent.

A connection that had lasted millions of years, gone in a moment.

And you haven't got to look very deeply

at the surface of our country to discover our European roots.

So, Europe is closer than you think.

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