Original subtitles

In the heart of the Americas...

..where continents collide...

..there's a land full of natural riches...

BIRDS HONK

MONKEY GROWLS

..a land of towering giants...

..scorching sands...

..and secret rivers.

Where great civilisations rose...

..and fell.

To succeed here takes passion and spirit.

This is a country rich in colour and culture...

..a festival of life.

This is...

In Mexico's far south there's a forest full of secrets.

It's March, the peak of the dry season.

This pool is fast disappearing...

..and it draws all the animals from the forest.

Even the most elusive.

For a young Morelet crocodile, the pool is also a hunting ground.

There's plenty of potential prey.

Trouble is, most are far too big.

Finally, a realistic target.

Even at his size, sneaking up in such shallow water isn't easy.

He needs to change his technique.

Perhaps an ambush will work.

Almost.

Success at last.

But he's not the only crocodile here.

And his taste of victory is short-lived.

There's a reason water is so scarce in this corner of Mexico.

This great forest, which stretches for almost 50,000 square miles,

has no major rivers.

And that's down to the region's unique geology.

The forest stands upon a vast peninsula, the Yucatan.

The entire peninsula is a gigantic slab of limestone.

Limestone is incredibly porous.

Any surface water swiftly drains away underground.

And this shapes the lives of all who live here.

For seven months of the year,

virtually no rain falls,

and by April, much of the forest is barren.

For a mother spider monkey this can be challenging.

She needs to produce enough milk to feed her baby.

And that requires plenty of fresh fruit.

Sometimes, that means travelling over two miles each day.

But with long, slender fingers and no thumbs to get in the way

this is child's play for an animal

designed for life in the tree tops.

She's taking her youngster to a special place...

..where she knows there's fruit all year round.

The ancient city of Calakmul,

once one of the most powerful in the Maya civilisation.

In the seventh century, this city was home to 50,000 people.

The people of Calakmul

cultivated trees that would fruit throughout the year.

Centuries later, these ancient gardens have left a lasting legacy.

There's more fruit here than anywhere else in the forest...

..even at the height of the dry season.

Over countless generations,

the monkeys have passed this knowledge on to their youngsters.

MONKEY CALLS

At its peak in the eighth century,

the Maya civilisation grew to almost 13 million people.

Theirs was a highly sophisticated culture, advanced in mathematics,

language and astronomy.

The huge limestone temples, built to worship their gods,

are a lasting testament to great feats of engineering.

Almost every other ancient civilisation in history

has been built beside a major river.

So how did the Maya manage without one?

There are almost 1.5 million direct descendants

of the ancient Maya living in Mexico today.

Don Roque and his wife Dona Su live on an isolated farm

in the heart of the Yucatan's forest.

GOAT BLEATS

This farm has been in Don Roque's family for generations.

With his children grown up and left home,

nowadays, it's just him and Dona Su.

They live off the land in a way

little changed since the times of their ancestors.

But there's more to their farm than meets the eye...

..a huge natural well, known in the Yucatan as a cenote.

A cenote is a collapsed cave.

Over thousands of years, rainwater has eroded

the soft limestone on the surface.

Eventually, the roof weakened and fell.

This cenote provides Don Roque with water all year round.

But it's far from unique.

It's one of several thousand cenotes scattered across the peninsula.

Cenote comes from the Maya word ts'onot,

meaning holes with water.

It was these mysterious pools

that enabled the Maya civilisation

to flourish without a major river.

And the greatest ancient cities were founded beside them.

Each cenote is an oasis.

The surrounding forest remains lush,

even at the height of the dry season.

Filtered through the limestone, the water is full of minerals.

It supports a unique community of plants and animals.

Some fish in the Yucatan cenotes are found nowhere else on Earth.

Within the dense forest,

a cenote can remain hidden, even when close by.

But the Maya had an almost sure-fire way of finding them...

BIRD CALLS

..the distinctive call of the turquoise-browed motmot.

Virtually every cenote has its own colony.

Elsewhere, motmots prefer to nest alone,

digging their burrows along river banks.

MOTMOT CALLS

Here in the Yucatan, they're forced together,

sharing limited space on the cenote's soft limestone walls.

Before a male can start looking for a mate,

he needs to secure a nest site.

But this cenote is already crowded.

17 other pairs are nesting here.

If he's to fit in, this new arrival must use the right body language.

MOTMOT CALLS

A raised turquoise brow is intended to intimidate rivals.

The pendulous tick-tock of their tails

has earned motmots the nickname clock birds.

This male must persevere if he wants to breed this year.

At last, he's earned his own little ledge of limestone.

With a gift of food,

he can get down to the business of attracting a mate.

There are so many cenotes in the Yucatan

because of a dramatic event

that changed the history of life on Earth.

Around 65 million years ago,

a huge meteorite collided with what is now the Yucatan's north coast.

Its effect was so catastrophic...

..it's believed to have wiped out the dinosaurs.

In an instant, the impact fractured the limestone platform,

creating what has become known as the ring of cenotes.

Satellites have revealed over 200 cluster around its crater.

Satellite technology is also helping modern-day explorers in the Yucatan.

And new cenotes are still being discovered today.

It should be about 50 metres that way. You see anything?

Sam Meacham and Fred Devos are part of an international community

of cave divers that has been exploring the Yucatan

since the 1980s.

I started out for the thrill of being able to explore something.

And from a young age, I just was imbued

with the desire to go out and explore this world.

Each new cenote adds another piece to the puzzle.

- Oh, look at this, Sam. - Wow.

The moment of discovery, right? You can't beat it.

In the last 30 years, cave divers in the Yucatan

have revealed something extraordinary.

The cenotes are not isolated wells.

Each is connected by an intricate network of caves and passageways,

spreading like a spider's web across the peninsula.

Divers have discovered over 350 caves

and nearly 1,000 miles of flooded tunnels.

Including the two longest underwater cave systems on the planet.

Many of these caverns have seen fewer visitors than the moon.

The great majority remains uncharted.

For the ancient Maya, the labyrinth of caves

was out of reach but not beyond imagination.

This secret underworld was held sacred.

Home to the 12 gods of Xibalba,

it was a place both feared and revered.

But the ancient Maya recognised their link to this unknown world.

Trees.

Always plumbed into the water below,

trees allow life above ground to flourish.

No tree is better equipped to do this than the strangler fig.

Its roots can reach 30 metres into the water below.

Don Roque uses the fig's roots to enter the underworld...

..navigating his way down steps carved by his grandfather

over half a century ago.

At the bottom of the cenote, Don Roque has a secret garden.

Here he cultivates a plant that wouldn't survive

elsewhere on his farm.

Coffee.

Above ground it's too hot and dry for coffee to grow.

Down here it's two degrees cooler and a lot more humid.

Don Roque's cenote is also a haven for insects.

Food for a colony of resident cave swallows.

By May the swallows are intent on one thing.

Breeding.

The cenote provides everything they need.

Down here they have a ready supply of soft mud.

They use this to craft their nests

together with fibres plucked from the hanging fig roots.

All being well, in around a month's time,

they'll each have a brood of up to five chicks.

Don Roque's cenote has its own community of life

thanks to the constant presence of water.

But the rest of his farm requires something more.

He plants his land with a mixture of corn, beans and squash

in a tradition passed down the generations.

A mix of crops helps maintain the fragile soil

which lies in a thin layer on top of the limestone.

But he can't draw enough water from his cenote

to irrigate an entire field.

For his crops to grow, Don Roque needs it to rain.

By late May, change is in the air.

The Yucatan relies on weather systems

that develop thousands of miles away.

Gathering ever more water as they cross the Atlantic,

they bring the seasonal rains.

The ancient Maya believed the rain was a gift from the gods.

For Don Roque, the old gods are very much alive.

RUMBLE OF THUNDER

The most important of all is Chaac, the life-giving god of rain.

It is Chaac who determines each thundercloud.

CRACK OF LIGHTNING

RUMBLE OF THUNDER

Finally letting the rain fall.

1,200 years ago, something happened that sent the Maya civilisation

into a dramatic decline.

Chaac withheld the seasonal rains.

The Yucatan was gripped by a series of devastating droughts.

Over 80% of the population vanished.

One by one, the bustling cities were abandoned

and reclaimed by the forest.

MONKEYS HOWLING

This year the gods have been kind.

By July, it's raining almost every day.

MONKEYS GRUNT AND HOWL

This brings new growth to the forest.

Good news for the Yucatan's noisiest residents.

LOUD HOWL

Howler monkeys.

Their calls carry for over a mile.

MONKEYS HOWL

They howl to let neighbouring groups know their whereabouts.

This patch of the forest is worth protecting.

The troop has found a tree with young, succulent leaves -

a particular favourite.

But a diet of leaves takes time to digest.

So howlers spend up to 80% of their time resting...

..making them amongst the most peaceful of all monkeys

despite their fearsome calls.

Baby howlers feed on their mother's milk until almost a year old.

This is much easier to digest.

With his mother trying to sleep off her lunch,

nap time can be a bit dull for a baby.

RUMBLE OF THUNDER

All life in the Yucatan benefits from the rain.

Trees burst into fruit.

All the new plant growth

provides food for an explosion of insect life.

There are over 3,000 different species on the peninsula.

The insects become food for others.

Almost all the birds in the forest schedule their nesting to coincide

with this seasonal bounty.

The regular rain is also good news for Don Roque.

In just six weeks his corn has shot up.

But the explosion of plant-eating insects

could cause Don Roque problems.

For his crops to thrive, he needs the help of a natural ally.

In the heart of the Yucatan,

there's a cave the locals call El Volcan de los Murcielagos.

The bat volcano.

Every day, an hour before dusk,

it erupts.

More than three million bats exit the cave.

The ancient Maya associated bats with death.

This was a swarm straight from the underworld.

But bats are vital to life in the Yucatan,

keeping its insect population in check.

Each can eat its own body weight in insects every night.

While the bats are focused on the insects,

others are watching them.

Grey hawks...

..and bat falcons.

These birds are skilled aerial predators,

their talons perfectly designed to snatch the bats from the air.

There are other more surprising hunters here.

Brown jays.

They normally eat insects and fruit.

But the brown jays here have developed a taste for bats.

The jays don't use talons to catch their prey.

They use their beaks.

The loss of ten or so bats barely makes a dent on the swarm.

Tonight, the colony will consume around 30 tonnes of insects.

A vital service that keeps the entire forest healthy.

It's late September...

..and it has rained almost every day for the last three months.

Don Roque's corn is now fully ripe.

In a tradition dating back to the times of their ancient ancestors,

the first corn of the harvest serves a special purpose.

Dona Su crushes it to make a sacred drink called atole.

This is offered to the gods in a ceremony of thanksgiving.

SPEAKS IN NATIVE LANGUAGE

From June to November, the Yucatan receives 100 centimetres of rain.

Across the peninsula, that's over 36 trillion gallons.

The cenotes are refilled.

But most of this water seeps through the limestone into the underworld.

The work of cave divers like Sam is furthering our understanding

of the Yucatan's unique water system.

'We're still exploring here.

'And the more we explore, the more questions we ask.

'Selfishly, I hope we finish in my lifetime.

'I don't think we will.'

Metre by metre, the divers are mapping the underground labyrinth.

It's a painstaking process with every measurement taken by hand.

But this has revealed something remarkable.

The water in the caves isn't just locked within the limestone.

It flows.

The Yucatan's missing rivers are underground.

Like all rivers, they are compelled towards the coast.

Here, the fresh water meets salt water brought by the incoming tide.

This border is called the halocline.

Containing more dissolved particles,

the salt water is heavier and lies below the fresh water...

..giving the illusion of a surface.

But after months of heavy rain...

..the flow of fresh water prevails.

At the coast, it leaves the underworld behind.

Mangroves flourish in the mixture of salt and fresh water.

The roots provide ideal nurseries for schools of fish.

And gentle giants are drawn to the Yucatan's shores.

Manatees come to drink at the outpouring of fresh water.

As the fresh water flows through the mangroves,

the trees release tannins, staining it brown.

Eventually the fresh water reaches its final destination.

It pours out into the Gulf of Mexico in huge volumes.

DRAMATIC MUSIC

Replenished by billions of gallons of fresh water,

these vast coastal lagoons are the ideal home

for a colony of the Yucatan's most colourful residents.

Caribbean flamingos.

It's been a good year for the colony,

with nearly 15,000 new additions.

Finding your chick in such a big crowd isn't easy.

FLAMINGOS SQUAWK

But flamingos have incredibly good hearing

and her youngster has his own unique voice.

Just six weeks ago,

the chick was little bigger than his mother's beak.

These lagoons are full of brine shrimps.

The beta-carotene in these tiny crustaceans

gives the flamingos their distinctive colour.

But it will be another two years before he starts to turn pink.

Here on the Yucatan's coast, as elsewhere on the peninsula,

all owe their survival to the secret underworld.

Without its great hidden rivers,

this would be a hot, dry and hostile place.

Born of a unique history,

the underworld hidden beneath this vast forest

gave rise to the ancient Maya civilisation...

..and it remains the lifeblood of the Yucatan.

Capturing the beauty of the Yucatan's underworld

meant sending a film crew into one of the most alien environments

on the planet.

To do this safely required expertise.

Between them, this team has over 50 years' experience

diving on the peninsula.

I want to make sure we get that right, you know?

Cameraman Mike Madden was one of the pioneers

of cave exploration here back in the 1980s.

And Sam Meacham is part of a team currently mapping

one of the region's two longest caves.

There's a common misconception that cave divers are these

adrenaline junkie risk-takers cheating death on every dive.

Were tarantulas in our risk management form?

'When, in fact, we're probably some of the most cautious people

'on the planet.'

There's a very real need for caution.

All diving carries risks.

But cave diving heightens these risks.

Surfacing isn't an option if you run out of air.

This particular cave system, known as Taj Mahal,

was first explored in 1995.

It has over three miles of flooded passageways,

some descending almost 30 metres.

Filming here poses another challenge.

Most of the cave is pitch black.

So the first thing the divers need to do

is set up a highly specialised underwater lighting rig.

Four 1,200-watt lights.

100 metres of cable.

All of it has to be carried into the cave by hand.

Lighting the caves on this scale is a new challenge for the team

and draws on all their expertise.

Since the 1980s, the dedicated work of divers like Sam and Mike

has helped put nearly 1,000 miles of the Yucatan's underworld on the map.

But the vast majority remains uncharted.

It's pretty amazing that 35 years after the beginning

of cave diving exploration in this area,

I would say we're still really beginning to understand

what's going on here.

And now there's an added urgency to their work.

Over the last 50 years the Yucatan's population has skyrocketed.

The city of Cancun is one of the fastest-growing tourist destinations

in the world, with over five million visitors every year.

This creates jobs, but the demand for fresh water

is putting pressure on the peninsula's underground rivers.

The future of this area is dependent on tourism

and so how do we sustainably manage these incredible natural resources

so that Mexico can continue to receive

10-12% of its gross domestic product through tourism

on a 100-mile strip of beach?

That's important to the future of Mexico,

not only of this region but the country as a whole.

Maps of the underground river systems are far from complete.

New construction work may be taking place above caves

that are yet to be discovered.

The entire city of Cancun draws its drinking water from 142 cenotes,

some of which are now at risk of pollution.

It's a race against time to reveal the importance

of the Yucatan's hidden underworld.

Back at Taj Mahal, with the lights in place,

the crew are ready to begin filming.

All our focus is going to be this direction today.

Every image they film must be meticulously planned.

Get over. More light.

Less light.

Once the dive begins,

all communication is through hand signals.

At last the team can capture the splendour of the hidden underworld.

For most of us, this flooded labyrinth

remains just as inaccessible as it was to the ancient Maya.

But for the cave divers devoted to exploring this world,

the hope is that by opening our eyes to its beauty and importance,

they will help safeguard its future.

In the next episode, we head to Mexico's remotest region.

The far north.

A land of great deserts and rich prairies.

Home to the toughest of characters...

..secret worlds...

..and some of Mexico's rarest wildlife.