NARRATOR: Lost civilizations. Mysterious sunken cities.

And legendary Atlantis itself.

Vanished beneath the waves.

PATRICK: There is a huge amount of unrecorded human history beneath the

ocean's surface that we don't know very much about at all.

NARRATOR: Imagine if we could empty the oceans, letting the water drain away

to reveal the secrets of the sea floor.

Now we can, using the latest underwater technology.

Piercing the deep oceans and turning accurate data into 3D images.

Is this a huge underwater temple carved by a mysterious lost civilization?

NARRATOR: Why are these perfectly circular stones

lying at the bottom of a bay in the Aegean Sea?

SIMON: There are shapes on the seabed that just don't make sense.

NARRATOR: Are these just rocks, or the remains of an ancient city?

JON (over radio): Parts of this are just beginning to be revealed for the first time.

Wow.

NARRATOR: And is the real site of Atlantis finally about to be revealed?

-People want to believe in places like Atlantis.

They want to believe in other worlds.

(waves crashing)

NARRATOR: Stories of sunken cities have fascinated for millennia.

The most tantalizing of all: Atlantis.

First described by ancient philosopher, Plato, a dazzling civilization

destroyed by the gods as punishment for human pride.

-People connect to that story, and it's because of this idea of an ancient civilization,

a utopia, that we've evolved from.

NARRATOR: 2,000 years later, the idea of Atlantis

continues to fascinate A-list archaeologists.

And the producers of B-list movies.

MAN (over TV): Atlantis is the story of those who, like today,

would use the marvels of science to conquer and enslave.

Atlantis, a world that worships strange gods of science,

of science, a science gone berserk.

-Plato would have loved this okay?

He would have had a great time.

He would have said, 'at last, you know, people are listening to me.

People are taking my story seriously.'

And he would have enjoyed this immensely.

NARRATOR: Using science, not science fiction, draining the oceans

exposes new evidence about Atlantis.

And other stories of great sunken cities, around the world.

As the waters recede from around the remote Japanese island of Yonaguni,

a mysterious formation begins to appear.

Could it be the creation of an advanced, ancient people?

Yonaguni lies in the East China Sea.

The waters here are perilous.

Schools of hammerhead sharks patrol, amid powerful currents.

Typhoons frequently batter the island.

For divers, it's the ultimate challenge.

Kihachiro Aratake is one of the pioneers of diving here.

KIHACHIRO: The water was crystal clear.

I felt as though I was looking down from the seabed from the sky.

NARRATOR: Searching for a new dive site, he makes a startling discovery.

-I saw a steps-like formation, like terraced fields.

I thought for a moment it was similar to the Machu Picchu ruins.

So, I named the spot the submarine ruins.

NARRATOR: It's an extraordinary spectacle.

The size of five football fields.

Smooth-sided walls rise up to the summit, the height of an eight-story building.

Flights of stone steps climb up from the base.

And on every side, the shapes and forms are strikingly regular.

-It can't be formed naturally, without human involvement.

It couldn't be like that.

NARRATOR: So, what could it be?

It's impossible to investigate the structure fully by diving.

The only way is to conduct a survey using the latest scanning technology.

HIRONOBU: Although the shallow waters around Yonaguni Island are close to human habitation,

we don't know much about them.

It's an unknown frontier.

NARRATOR: Using sonar scans, Hironobu Kan records a

complete digital map of the sea floor.

Using this data, it's possible, for the very first time,

to drain away the waters and reveal what lies beneath in exact detail.

The summit of the mysterious structure begins to appear.

(waves crashing)

Water pours off its smooth terraced layers, exposing them once again to the sky.

And with the monument left high and dry, its true scale becomes clear.

Vast, imposing and spectacular.

The incredible discovery attracts worldwide attention.

Divers, journalists and TV crews flock to Yonaguni, all asking:

what could this mysterious structure be?

And scientists come here too.

Among them is Professor Masaaki Kimura, one of Japan's top marine geologists.

KIMURA: When I dived underwater to explore it I felt right away that it must

be a man-made structure.

NARRATOR: He finds several features that he believes indicate human activity.

-Here, you'll see something that looks like the front gate of a castle.

It's like a tunnel, and you go through it.

NARRATOR: On the far side of the tunnel, a road loops around to a flight of steps.

Watched over by two mysterious stone objects.

-There are mound-like bulges here and here, and if you look at them,

you'll find turtles with their necks extended, on both sides.

NARRATOR: According to ancient Japanese folklore, the dragon god, Ryujin,

lives in an underwater palace, watched over by turtles.

To Professor Kimura, this indicates that the monument

is so important it needs guarding.

And that's not all.

-If you go up this path you get to this place.

This portion is triangular.

That's why it's called the triangle pool.

Since this is fairly big and shallow as well, you can relax in it.

Also, the sea surface is very close to it and it's warm due to the sunlight.

You feel chilly after you come out of the deeper area down there,

and you can warm up here.

NARRATOR: But the monument's most striking feature is its wide, flat terraces,

Large enough to support thousands of people.

Professor Kimura believes the gateway, stairs, turtles and terraces

all point towards one purpose.

-Judging from its shape, I think they used it as a castle.

NARRATOR: But to prove his theory, Professor Kimura needs more:

the artifacts of those who once lived here.

He scours the structure looking for clues, and

discovers what look like primitive tools.

Then, hidden behind one of the great walls, the most remarkable find of all.

-Look at this, a symbol is inscribed here and here's a hole.

Probably, they hung this article like this.

And here's something looking like a letter.

Maybe it could be a kind of a talisman to protect people from evil.

NARRATOR: For Professor Kimura, the case is closed.

The artifacts and the monument's regular shapes prove that humans lived here,

8,000 years ago.

After that, rising sea levels claim the territory of this lost race of master builders.

But is that the real story of Yonaguni?

NARRATOR: Professor Kimura believes that this extraordinary structure

off the coast of Japan is a gigantic castle, built long before the pyramids of Egypt

by a mysterious lost civilization.

But is it?

With the water drained away, it's possible to scrutinize the monument in forensic detail.

The edges appear regular and straight, as if made from blocks carved by human hand.

But closer examination reveals something else.

The surfaces are smooth and unbroken.

The monument is not assembled from handmade blocks,

but is in fact one solid mass of rock.

HIRONOBU: The pyramid area and the headland behind it are connected with each other.

The surface of the terrain has a lot of grooves, so you may think they are separate,

but since they consist of the identical stone, they are originally connected.

NARRATOR: The basic form of the monument is clearly natural.

But what about the gateway, steps, turtles and pool?

Could it be that an ancient people carved into this huge rock,

transforming it into a great castle or temple?

Studying the headland on shore provides the answer.

The shapes here are strikingly similar to those found on the monument,

and that's because they were formed in exactly the same way.

Both are made of sandstone which, when subjected to stress,

such as during an earthquake, can fracture along vertical faults,

forming angular shapes and what look like steps.

PATRICK: Put something like that underwater, have the water wash over the top of it

and clear away all the debris, and you've got very, very fine step-like structures.

But they're no more human made than any other structure down there.

NARRATOR: The lack of tool marks is further confirmation.

The forces which shaped this remarkable place are geological.

But what of the artifacts?

Perhaps the monument, if not built by ancient people, was inhabited by them.

Walking these steps.

Crowding these natural terraces.

But so far, no other objects have been found, suggesting that the talisman and tools

were dropped from a boat passing overhead, landing on the monument simply by chance.

-The clear thing for Yonaguni for me is there's no pottery,

there's no evidence of actual human occupation.

There's not a single wall from the site.

There's nothing on it, that indicates human activity.

NARRATOR: The Yonaguni Monument is an extraordinary natural formation,

created by epic geological forces.

But its shapes appear so regular that many still believe

it holds a secret, ancient purpose.

JIM: You can take that leap if you have an imagination and say yes,

this could be an ancient city beneath the sea but in the case of Yonaguni

I'm in those ranks that feel that it's not.

It's geology.

-We want to believe that science doesn't have all the answers.

We want to believe innately that there is something out there that we can connect with

spiritually and that hasn't really been polluted by

being the subject of scientific discovery.

NARRATOR: And still amongst the believers is Professor Kimura.

He remains convinced that the monument was inhabited by his ancestors and continues to

search for evidence to prove the doubters wrong.

Beneath the oceans of the world lie many more

tantalizing traces of possible lost civilizations.

As the water continues to drain away, in the Eastern Mediterranean,

astonishing structures emerge in a quiet island harbor.

Is this jumble of shaped stones the architectural debris of a once great city?

Alikanas Bay, a tourist hotspot on the island of Zakynthos, Greece.

Diver, Pavlos Voutos, sets out to take some underwater photos.

NARRATOR: Pavlos swims farther out into the bay in search of clearer water.

Then, out of the gloom, he sees something that will change his life.

NARRATOR: The debris stretches out for hundreds of yards in all directions.

The area is so large that Pavlos is convinced he's found the remains of an entire town.

NARRATOR: The discovery sends a bolt of electricity through the world of archaeology.

Professor Michael Stamatakis, immediately travels to Zakynthos to investigate.

NARRATOR: Stamatakis recalls seeing similar shapes on land,

at the site of an ancient settlement built over 2,000 years ago.

NARRATOR: If the same shapes lie on the sea bed, they could indicate an ancient settlement

just offshore concealed under the bay and forgotten about for centuries.

Comparing the images is not enough.

The only way to reveal a complete picture of the structures

is a detailed underwater survey.

Simon Brown is an expert in 3D modelling.

But the task facing him is immense.

SIMON: Right now we're not quite sure how big the area is.

I've estimated it's about 16 acres, which will be

more than double the largest subject I've ever covered to date.

It's a weird place.

There is definitely features here that I have never seen anywhere else before.

They look out of place.

But then I started to see more regular shapes that looked that cut stone.

So my mind then starts to think, is it...

is it manmade?

NARRATOR: Can draining the waters of the Mediterranean provide the answer?

NARRATOR: Simon Brown is mapping mysterious

underwater structures discovered in a Greek bay.

Could they be the remains of an ancient town?

He takes nearly 4,000 high resolution photographs of the sea floor tracking each with

pinpoint accuracy through GPS.

Using these images, it's now possible to do something which has never been done before:

drain the waters of Alikanas Bay and reveal, for the first time ever,

a 3D plan of the entire sea floor.

As the Mediterranean begins to recede, a world is exposed

that's been invisible for thousands of years.

First, the rocky shoreline is left high and dry.

Then, from the dark depths, regular shapes begin to appear,