Original subtitles

Hidden in this jungle are 3 ,000 years of human history, one of the world's

greatest ancient civilizations.

Here, on Mexico's Yucatan Peninsula,

the Maya built cities, temples, and palaces, and yet we still don't know how

they thrived.

The forest has grown back, and nature has taken over again.

leaving many riddles unsolved.

Like the riddle of the missing river.

Almost every other ancient civilization was founded beside a great river.

But there are none here.

Not even any streams.

Where is the Nile?

The Ganges or the Euphrates of the Maya?

What they did have were thousands of these pretty little pools scattered

the jungle.

Called cenotes, they're the Yucatan's only source of fresh water.

Could they by themselves have supported an entire civilization?

The Maya believed that cenotes were entrances to another world.

An underworld.

but faith value they seem to be little more than beautiful jungle water holes

was the underworld just a myth people today can do something the maya could

have dreamt about breathe underwater these modern explorers have made some

remarkable discoveries not only about the maya but about the forest and its

animals too what they found in the underworld

has changed our understanding of the Yucatan forever.

The Yucatan, a peninsula the size of England, separating the Gulf of Mexico

the Caribbean Sea.

American -born Sam Meacham is a cave diver.

He's been exploring the waters under the Yucatan for more than a decade.

But he's still only seen a fraction of what's down there.

His mission is to explore as many cenotes as he can, working with

try to make sense of it all.

The puzzle of the Yucatan Peninsula is extremely complex. I arrived here in

with the intention of only being here for six months, and ten years later I

myself still here, so interested and curious in what I've discovered.

But Sam wasn't the first foreign explorer to be drawn to Mexico's jungles

passion for adventure.

Back in 1839,

John Lloyd Stevens, an American diplomat and travel writer, set off into the

Yucatan, inspired by rumors of a lost civilization.

For a while, he found nothing, even though clues lay all around him.

Finally, he stumbled upon the ruins of a great city, smothered by the jungle.

The wild tales that Stevens told made his name as a famous Victorian explorer,

hero of his time, and to some, the original Indiana Jones.

Stephen's fantastic revelations have inspired a whole new generation of

explorers.

For me, one of the great motivating factors in what we do here is that I'm

to explore in the 21st century, something I thought would never have

possible in my lifetime.

Just getting to the cenotes is an adventure in itself.

Like the Maya ruins, they're scattered over thousands of square kilometres of

trackless forest.

But Sam's not alone.

British -born Steve Bogarts shares Sam's passion for exploration.

They've been cenote hunting together for years.

With local help, they mount expeditions deep into the Yucatan's interior.

It can take days to find a new cenote.

As we travel through the jungle looking for cenotes, of course there's always

the usual assembly of spiny trees and cactuses. We have crocodiles, we have

snakes, scorpions, tarantulas, you name it, it's all there.

But really, if you know what to look for and know where to go and where not to

go, you can avoid a lot of these problems.

Finally, a new, unexplored cenote.

Never mind the jungle tracks, the real danger for Sam and Steve begins at the

bottom of these enchanting little pools, considered sacred by the Maya.

It's very easy to see how the ancient Maya would have perceived the cenotes as

very sacred spaces. They're absolutely beautiful jewels out in the middle of

this jungle.

To walk up to the edge of a cenote and to look down into the crystal clear

and see the fish swimming below in the natural daylight, casting these

incredible shafts of light through the water, is very inspiring.

As the sole sources of water in this jungle, these pools are also magnets for

wildlife, and to cenote specialists like Greaves, their whole world.

With thick forests on all sides, they seem as isolated as islands in an ocean.

Peccaries, deer and other forest animals use cenotes as watering holes.

But that doesn't mean they're easy to see.

The jungle.

dug its best to keep them hidden.

But some animals you can't help but notice.

Howler monkeys.

Even if you don't see them at first you are sure to hear them.

With calls that carry five kilometers, they're the loudest land animals in the

world.

Howlers are sloppy eaters.

Coates following below can fill their stomach solely out of what they've

dropped.

Spider monkeys.

They're infinitely quieter than howlers, but much more agile.

With their hooking hands and long arms, these monkeys can live their entire

lives in the dense forest canopy.

For nine months of the year, there is no rainfall here, and much of the forest

struggles to survive.

But some trees seem immune to the drought.

What's their secret?

Such are the riddles of the Yucatan.

The answers lie underground.

But Sam and Steve won't get to go there until tomorrow.

In the dark, the jungle seems even denser, and the sounds even stranger.

This is when cenotes really come alive.

Tapirs love water, for bathing as much as drinking.

But visiting a cenote means coming into the open, which, for good reason, they

only do after dark.

Like watering holes anywhere, Cenotes are where predators, in this case

come to hunt.

But to the Maya, cenotes were more than just jungle watering holes.

They were central to their world.

Cities and temples were often built right next to them.

These sacred wells were gateways to the underworld, a terrifying place of

spirits and of fearsome gods who demanded respect.

At the bottom of many cenotes lie offerings made to the underworld.

For archaeologists, cenotes are time capsules that provide clues to how the

ancient Maya lived and died.

Sometimes even the people themselves were sacrificed to the gods they feared

much.

Every pot and skeleton has its own story to tell.

The discoveries of underwater explorers are helping archaeologists rewrite

the Yucatan's ancient history.

Yucatan's explorers aren't just interested in the clues to Maya history

they might find at the bottom of these pools.

They want to know what might lie beyond them.

Is there indeed an underworld?

Could this cenote be a gateway to a whole new world?

If it is, where does that world lead?

Every new cenote...

presents a new opportunity.

Cenotes really present us with the truest form of exploration found today.

we come up to the side of a cenote, we literally have no idea what we're going

to find at the bottom of it until we actually get in and investigate.

And for me, that's one of the greatest thrills about what we do.

Ready?

Almost.

Cenotes aren't just simple pools.

They're caves.

Flooded caves whose roofs have collapsed.

But Sam and Steve have yet to discover to what extent cenotes are connected to

each other by flooded tunnels.

If there is a network of tunnels down there, how far does it go?

What they're doing is carefully charting an as yet uncharted part of the planet,

somewhere no other human being has ever gone.

It's one of the riskiest things an explorer can do.

This type of diving isn't for everybody.

and definitely you have to want to do it in order to be involved in it.

The first cave dive that I ever did, actually, I was pretty nervous.

I talked to an astronaut that sat on top of a rocket full of fuel and blasted

off to the moon.

Sure, I bet they were a little bit nervous, but look what we've gained

space exploration.

All those people were willing to take a risk to achieve an incredible goal.

Sometimes there's hardly enough room to squeeze through.

Getting stuck or damaging vital equipment now would be fatal.

We're diving in an extremely hostile environment. It's underwater, it's dark,

it's easy to get disoriented, and therefore it's easy to have panic

There's two ways out of a panic situation, luck and death, and therefore

is not an option for us.

You really have to take three deep breaths, calm yourself and assure

that you're able to get out of that situation.

Exploration is rarely without risks.

But one of the biggest rewards is seeing something that's never been seen

before.

What they've discovered down here is just staggering.

The Maya did have an underworld.

And it's as strange and as beautiful a place as any myth might describe.

They've revealed a vast system of flooded caves, underpinning much of the

peninsula.

It's changed our view of the Yucatan forever.

In a sense, this is like exploring outer space.

The weightlessness, the utter strangeness, the thrill of the unknown.

Cave divers call this inner space.

Sam has got close to a long -held ambition.

One of my childhood dreams was to become an astronaut.

I'm not an astronaut now, but I feel that I'm as close as I can come to outer

space exploration in the work that we do here.

We're completely dependent on life support equipment. We travel into a

completely alien and foreign environment that we don't know a whole lot about.

And many of the cave systems that we dive in have seen fewer visitors than

surface of the moon.

It's amazing to think that a whole civilization once sat on top of all

trying to imagine what was down here.

The reality of this place can be as surreal as anything that Maya may have

dreamt of.

Sometimes, what seems to be air, isn't.

It's just a different kind of water.

Some caves contain layers of water that just don't mix.

There's so much about this system that we don't yet understand.

Sam and Steve's aim is to find out how it all connects.

They're making maps.

Light ahead reveals a new cenote.

They'll record its position then swim back to where they started to dive and

to return here overland The

more they explore the more connections they find

But they've got a long way to go There are still thousands of cenotes left to

investigate

The return journey is in many ways more difficult.

GPS puts us right on target.

Underground, they went where the tunnels led them.

Up here, they're looking for one tiny pool among thousands, hidden somewhere

a dense jungle.

For this, they'll need satellite positioning and aerial photographs.

Yeah, GPS puts us right on it, huh?

Yeah, that's promising.

State -of -the -art technology gets them close, but on the final stretch, they

get a helping hand from birds.

Turquoise -browed motmots.

These are true cenote birds.

They feed on the abundant insects near the water and often nest inside the

caves.

Their distinctive call almost always means there's a cenote nearby.

It was the ancient Maya who first used them as guides to water.

This works just as well today.

Now they've located the new cenote, Sam and Steve need to find out if it has

further connections with other parts of the system.

In our corner of the Yucatan Peninsula, the collaborative efforts of cave diving

explorers have mapped and explored over 550 kilometers of

underground, underwater passageway in over 100 different cave systems.

The promise of future exploration is high. There's so much left that we still

have yet to explore.

This may seem like nothing more than an elaborate game of join the dots, but

each time Sam and Steve go back underground, they never lose sight of

potential dangers of their work.

One of the truisms of cave diving is that complacency breeds death, and every

single dive we approach is as if it's the first dive we've done.

And we have a ritual that we go through of matching our gear, checking for

leaks, and making sure that everything is in optimal 100 % condition for

Sam couldn't have a better dive buddy than Steve.

He's one of the region's most experienced cave divers and a master

He knows his equipment inside out.

Is everything good here? Yeah, looks good.

Okay.

One of the first things you'll notice is that we're actually taking two tanks

with us rather than one. That's because we're diving in an alien, potentially

hostile environment, and we need redundancy in all our life support

and gas supply is obviously very, very critical to us.

We also use a gas management planning rule known as the rule of thirds. So

use one third of our gas swimming into the cave, one third swimming back out

again, so that when we surface, we have one third in reserve. And that's an

emergency reserve should it take us longer to exit than we anticipated or if

needed to share air with a buddy.

A thin piece of white string, carefully laid.

quite literally becomes their lifeline.

It may be the only way that they can find their way back out of the

They

mark

it with arrows that always point back towards the entrance and safety.

It's also a measuring tape.

Regularly spaced knots tell Sam and Steve how far they've gone.

As we explore the cave systems, we try to be as smart as we can.

And generally, we're trying to go in a particular direction. And we have

compasses that work underwater.

And using those compasses, we're able to determine which route to take.

It's quite common to come up to a split in a passageway.

We have to determine which is the best route to take.

In some cases, that'll end up in a dead end, and we turn around and come back

out and try the other way.

Using spools of string, Yucatan's cave divers have measured the longest

underwater cave in the world.

over 133 kilometers long.

Exploration wouldn't be exploration if everything always went to plan.

This time, the divers have come to a passage too tight to squeeze through,

they're forced to stop.

They follow their safety line back and live to dive another day.

But explorers wouldn't be explorers if they let such setbacks discourage them.

There's always the thrill of the next dive.

It's pretty much guaranteed that every time that we go into a cenote, it's

to be a different experience. It's something new, it's something exciting,

that's what really draws me in.

One of the many interesting things of diving here is to watch all the wildlife

that thrives in the crystal clear water.

That includes sailfin mollies.

small fish that stick to the bright sunlit zones in the open water pools of

Cenotes.

For a male, it's a hectic life.

He has a three -dimensional territory to patrol and is constantly chasing other

males out while trying to keep his harem of females in.

In both cases, success depends on how effectively he displays his sail fin.

It's a big job for a little fish.

Some fish like these tetras have proved to be real opportunists.

They've learned to follow divers' torches into the dark to feed right

caves.

Our divers take care not to bring any uninvited guests with them, because the

underworld has its own unique creatures, an entire food chain of over 30 species

that live out their lives in the pitch dark.

Most cave animals are white, because in a world without light, colour is

pointless.

Even eyes are useless, and many creatures just don't have them.

Down here, touch and smell are all that matter.

Among the strangest and most ancient of cave beasts is the remipede, a sort of

primitive centipede that's rarely seen, found only in waters exceptionally low

in oxygen.

Relics of one of the earliest chapters of life on Earth.

They're among the cave's top predators, combing the water for shrimps and

isopods.

If the remipede doesn't seem to know which way is up, that's because in the

water and in the dark, up and down aren't so relevant.

In the underworld, even the fish are surreal, ghostly white, with blanks

eyes should be.

There are other signs of life down here.

This is the perfectly preserved tooth of a Gomphotherium, a relative of the

elephant that's been extinct for 10 ,000 years.

Ancient animal remains, and these stalactites and stalagmites only ever

in air, are hard evidence that these caves used to be dry.

And Yucatan's history goes deeper still.

The walls of these caves are made of soft limestone, telling us this was once

huge coral reef.

Some caves near the surface have air pockets and cracks in their ceilings

allow bats to come and go.

Cave swifts, too.

It's the perfect sheltered place to roost and nest.

No wonder the Maya thought bats were from the underworld.

They would have seen them flying straight out of the ground as night

By exploring underground...

Yucatan's divers are peeling back the many layers of the peninsula and are

slowly revealing the incredible relationship between its flooded caves

everything they affect at the surface.

There are many ways in which these two worlds connect.

Tree roots.

This is the jungle's secret. How, with hardly any surface water, it can still

grow so dense.

Some trees and vines push their roots through gaps in the limestone to the

permanent water supply below.

It doesn't matter how dry it gets on the surface, they rely on the underworld.

These deep -rooted trees provide animals with a year -round supply of leaves,

flowers and fruit.

This vital connection between the forest and the ground beneath it must have

intrigued the Maya.

It could only have reinforced their belief in the power of the underworld.

They too relied on its gift of water.

A few cenotes could help a whole city survive even the harshest of dry

But Sam doesn't just look to archaeology for his understanding of the Maya.

He can talk to them.

Direct descendants of the ancient Maya still live here.

One of them is Don Femine Zip, a good friend of Sam's.

The Maya still practice flash and burn farming.

growing crops than letting the forest grow back to replenish the soil.

In fact, the ancient Maya did this on a grand scale.

Incredibly, most of the jungle here, previously thought to be pristine, has

actually been cut down and regrown many times over the last 2 ,000 years.

The Maya may have stopped building large cities and temples, but they live on

today as skillful farmers, thriving.

despite the thin soils and harsh seasons of the Yucatan Maya

communities

are close -knit and the Mayan language is still spoken

Sunote is derived from the Maya word for well.

Almost every village is built around one.

Other sunotes mark boundaries between the communities.

The notice were and are quite literally central to their world.

As well as a distinct language, the Maya have a distinct set of beliefs.

Their stories and fables pass down the generations, describe everything around

them. The cenotes, the jungle, the animals.

One Maya belief is that the powerful forces of the underworld determine their

prosperity and their destiny.

Don Fermin still practices the Maya religion.

He prays to the gods of his ancestors and regards cenotes as windows into

world.

In advance of Tham's more difficult dive, Don Thamin sometimes makes

to the underworld, asking for a safe passage.

And this dive will be difficult.

But it will reveal yet another twist in the Yucatan's many -layered history.

A cosmic event.

It affected not only the world of the ancient Maya, but possibly the rest of

world as well.

Some cenotes near the northwestern tip of the Yucatan aren't at all like the

Sam and Steve are used to exploring.

These are much deeper, sheer, vertical sinkholes, known as pit cenotes.

This cenote is definitely a lot deeper than ones we normally would encounter.

Today we got to about 45 meters of depth and still we couldn't see the bottom.

This appears to be the bottom.

But it isn't.

It's a cloud of hydrogen sulfide made from rotting vegetation.

It's toxic.

and corrosive not somewhere you'd want to hang around

the

hydrogen sulfide layer is actually pretty intense as you're descending down

the cenote it gives the appearance that you're coming up on the floor and all of

a sudden you realize it's not the floor it's a cloud it's made up of sulfur

primarily so it's got a rotten egg smell to it In extreme cases where it's very

strong, you can feel it burning any exposed skin that you have.

Why are these cenotes so different?

They're evidence of a critical turning point in the Yucatan's distant history.

Something that was only noticed 20 years ago, when satellites gave us a new

perspective on life on Earth.

If you look at normal cenotes from space, their pattern is scattered and

But the pit cenotes form a distinct semicircle.

165 kilometers across.

Seismic studies have shown that the circle is completed under the sea.

So what does this huge circle represent?

The answer lies at least 65 million years ago, when the Yucatan was a

tropical sea.

The disastrous event that caused the circle was so massive that some think It

could have led to the demise of the dinosaurs.

An enormous meteor, heading for what is now the very tip of the Yucatan

Peninsula.

Imagine at the moment that this meteor slammed into our planet, it was so huge

that if one edge of it was touching our planet, the outer edge of it would be at

the same altitude as a commercial jetliner flies today.

The immense impact crater was gradually buried under limestone, built up by

coral reefs over millions of years.

But the crater's shape...

was echoed in the way this limestone then eroded to form the distinctive

semicircle of pits and otus.

When the Maya arrived, they built great cities and temples around these sacred

wells, unwittingly outlining the footprint of this global catastrophe.

Once again, the Yucatan's history can be read by looking deep into its

landscape.

But it has one more secret to reveal One last riddle to be solved

When

it does rain here, it rains hard

But this huge amount of water

doesn't settle on the ground.

It vanishes.

It seeps through the limestone into the underworld.

But this freshwater is only the top layer.

It floats above an enormous body of much heavier saltwater.

This is the halocline, the interface between the two.

It's this contrast between the gin -clear freshwater and the hazier

that can make diving here so surreal.

Divers have discovered that the freshwater here does more than just

It flows in huge underground rivers.

Probably the largest underground river system in the world.

Nearly two centuries ago, John Lloyd Stevens rediscovered the Maya

People have long wondered how they thrived without a great river.

Now, we appear to have found their Nile.

These great rivers must flow out to sea, but where?

Sam needs to find out.

He comes across the skeleton of a manatee, a sea mammal.

He must be getting close.

Meter by meter, cenote to cenote.

Cave divers are mapping the rivers from source to sea But

while doing so they've made an alarming discovery

The

modern world is taking over

I'm amazed at the changes

that have taken place in such a short time in this area. It seems that every

time I go out my door, there's a new building that's been built.

The coastal strip of Cancun and the Riviera Maya is one of the fastest

tourist areas in the world.

There's one specific occasion where we're actually diving beneath a major

construction project.

And as we were diving along, the entire cave was literally shaking as we were

diving through it.

And it wasn't until the next day that we came back.

that we realized that they had been perforating through the ceiling of the

and along one of the lines that Steve had laid the previous day, there was

actually a cement piling going right down through the cave system.

New construction could inadvertently block or pollute the great underground

rivers of the Yucatan, with far -reaching effects still too complex for

understand. The Maya underworld faces a new chapter in its long and varied

history.

The decline of the ancient Maya could teach us a thing or two.

Some say they developed too far too fast.

Others, that a succession of droughts left them without water.

Everyone here still relies on the underworld.

It is and always was the lifeblood of the peninsula.

Without it, the Yucatan would be a hot, dry and hostile place.

By mapping the course of every river to the sea, Sam and other divers are hoping

to draw attention to them, so further damage can be avoided.

Their work has not only helped us understand the Yucatan's past, but it

to safeguard its future.

Sam's journey down this river is nearly over.

There's more light and more air.

And the roots are roots of mangroves.

And there are manatees.

These gentle herbivores come to the underworld outflow to drink fresh water

to cool off.

What they mean to Sam is that he's made it.

One last tunnel.

And a journey that began in a jungle pool ends up off a Caribbean beach.

Tomorrow he'll be back in the forest, looking for a new cenote and the next

river. And when all the cenotes are explored and all the maps are finished,

maybe the Yucatan will be better understood.

In a more mystical way, The ancient Maya understood it.

They knew they were at the mercy of the underworld.

At the ruins, archaeologists are revealing ever more about this great

civilization, how they lived and what they believed.

But now, a whole new frontier has opened up underground.

Sam and Steve are not the first explorers to have been enchanted by the

of the Yucatan, but they have quite literally taken exploration to a whole

level.

To this day, it's only thought that we've charted a fraction of the Maya

underworld, and many of these areas still remain untouched and uncharted.

Sam continues with his passion.

He certainly has his work cut out for him in the coming years.

My feelings about exploration can be very easily summarized in a poem that I

read about the Yukon Gold Rush. And in that, the author says, it's not the

it's finding the gold.

It's finding the cenote and diving down into it and seeing what's there that

really is the thrill for me.

Really, for all of us, it's a motivation to think that you can live in the 21st

century and still be able to explore.

We're only just scratching the surface of what exists here. I have absolutely

doubt that this place will continue to provide incredible scientific

for years to come.

Sam and his explorer colleagues have risky yet fascinating days ahead of

unveiling the many secrets of the Maya underworld.