Undiscovered Vistas

The Yucatan Rainforest,

After the Amazon,

it's the largest rainforest in the Americas.

But there's more to this jungle than meets the eye.

Beneath the forest floor lies a mysterious underworld that holds

the secret to the rise and fall of an ancient civilization.

Hidden rivers feed vast marshlands that burst with life,

and lead to wild hunting grounds,

for perfect predators at the top of the food chain.

This extraordinary underground labyrinth breathes life

into the jungle.

Where caves erupt in a nightly feeding frenzy

and elusive animals fight to survive...

...in a lost world among the trees.

It's another dry day in the Yucatan.

For more than half the year,

virtually no rain falls in the forest.

And that's a challenge for this juvenile spider monkey.

He needs fresh fruit and leaves to thrive

and each day he must travel farther to find them.

Luckily, spider monkeys are perfectly designed

for life in the treetops, more than 100 feet above the ground.

Their tails can grab any branch with astonishing precision.

The tail serves as a fifth limb.

Enabling the monkey to even hang upside-down while foraging,

freeing its hands to feed more efficiently.

The little juvenile traverses the canopy,

searching for fruit and waiting for the rains to come.

The rainforest is the beating heart of the Yucatan Peninsula,

a vast coastal region of southeastern Mexico

that separates the Gulf of Mexico from the Caribbean Sea.

It's the largest continuous expanse of tropical rainforest

in Central America and extends from the northern end

of the peninsula, south to Belize and Guatemala.

The Yucatan boasts more than 700 miles of rich coastline

and is home to the largest barrier reef in the Americas.

The Mesoamerican Reef stretches for 70 miles

and provides a haven for dazzling coral,

fierce barracudas,

powerful nurse sharks,

and endangered hawksbill sea turtles.

About 65 million years ago, an asteroid the size of a mountain

crashed into what is now the Yucatan Peninsula.

The collision formed a crater nearly 150 miles wide

and more than eight miles deep.

No longer visible, the crater lies half-buried

under a blanket of rainforest.

The other half under the ocean.

The impact of the asteroid

triggered a mass extinction event that wiped out

the dinosaurs and three-quarters of all life on Earth.

Life had to begin again.

Today the Yucatan is home to more than 350 bird species

including the mangrove warbler whose lilting song

echoes along the coast.

It's a biodiversity hotspot where evolution has run wild.

What makes this region even more extraordinary,

there are incredible secrets hidden beneath

the forest canopy.

In these lush, rolling hills what appears to be another ridge

draped in jungle is in fact evidence of a Lost Empire.

An ancient Mayan pyramid.

This was one of the most important centers

of the Mayan civilization: Calakmul.

Its crumbled remnants stretch across nearly 10 square miles.

Jungle has all but devoured the ruins.

Moss cloaks almost every surface.

Giant trees lay silent claim

to what once was a booming metropolis.

The capital of the Kingdom of the Serpent.

At the centre of this once-powerful empire

is an enduring testament to ancient Mayan engineering:

Calakmul's great pyramid.

The Maya were the most-prolific pyramid builders in history.

For every pyramid the Egyptians built, the Maya built 10.

Each was designed as a sacred mountain

that housed an entry to the Mayan Underworld.

This towering pyramid, built with stone tools

over more than 12 centuries, was a Mayan gateway to immortality.

A place to talk to the Gods.

The ancient people of Calakmul cultivated trees

that bore fruit year-round.

Centuries on, these carefully designed orchards

continue to reap rewards.

For those who know where to look.

Which explains the racket in the trees.

The silence of the forest is broken

by one of the loudest sounds in the animal kingdom:

the cry of the howler monkey.

Few calls can penetrate the jungle as forcefully.

When a number of howlers bellow in concert,

the din can be heard up to three miles away.

Howler monkeys have large throats

and specialized shell-like vocal chambers

that help pump up the volume on their booming calls.

For those in the forest canopy,

there's no better way to stake your claim to territory.

Their howl serves as a warning to ward off other monkeys

to prevents skirmishes over food.

Because this troop has found its Eden.

The trees burst with fruit and succulent leaves,

a tasty favorite.

In the nearby treetops, the juvenile spider monkey

heeds the call of the howler and beats a hasty retreat.

This patch of forest is taken.

He'll need to look elsewhere.

All this noise and eating can be tiring.

When not foraging, howler monkeys spend up to

80 percent of their time lounging in the treetops.

That makes them one of the most peaceful species of monkey.

Their bark is worse than their bite.

Today, the ruins of Calakmul

are home not to the Maya civilization,

but to a family of iridescent osellated turkeys.

A species endemic to the Yucatan.

The city was left to decay after the collapse of the Maya

more than 1,000 years ago.

All that's left to speak for them

are mysterious images carved in stone.

These hieroglyphs were the history books of the day.

Raised to glorify kings, record royal births and weddings,

and honor military battles.

By the mid-6th Century, Calakmul was locked

in a bloody conflict with the neighboring city of Tikal.

In 695, Tikal struck, led by a young king whose name meant

God That Clears the Sky.

Calakmul raised its Snake army to counter the Tikal attack.

But the snakes were crushed.

So too were the dreams of an eternal Snake empire.

Still, it was in this ancient city that the snake dynasty

rose to its fullest glory as the largest and most powerful

Maya kingdom ever.

But what caused the fall of the Maya civilization?

Something led the people of Calakmul

to abandon their magnificent society.

After centuries of cultural, intellectual

and agricultural evolution.

Clues to the mystery lie even deeper in the jungle,

beyond the overgrown ruins.

Although this is a rainforest, water is scarce.

There are no major rivers, lakes or even streams.

Powerful rivers surge through most of the

world's great jungles.

So why not the Yucatan?

The answer comes down to its unusual geology,

found nowhere else on earth.

This forest sits on a thick bed of limestone,

once part of an ancient undersea reef.

Limestone is extremely porous.

It quickly absorbs rainwater,

which drains through the rock and disappears underground.

Small creeks, fed by seasonal rains, escape underground too

into the limestone caves and sinkholes that pock the Yucatan.

Throughout history, ancient people

have built almost every great civilization by a major river.

So how did these Maya thrive without one?

The key to their survival lies deep underground.

What the Maya believed was a gateway to the underworld:

the cenote.

The Yucatan is pitted with thousands

of these natural wells.

It takes millions of year for a cenote to take shape.

Over time, rainwater made acidic by carbon dioxide

in the soil percolates down into the earth,

eating away at the limestone to create a sinkhole

that descends to the water table.

Eventually the roof of the cave weakens and falls in,

revealing a secret realm to the world above.

Beneath the dark surface is an ecosystem

unlike any other on earth.

The cenotes are not isolated flooded caves.

Each is connected by an elaborate web of tunnels

that extends underneath the peninsula.

Over the past 30 years, divers exploring these

underground passageways made a surprising discovery:

the water isn't stagnant in each cenote.

Instead, it flows through the caves.

The Yucatan's lost rivers are still running underground.

More than 1,000 miles of tunnels course beneath the surface,

including the two longest underwater cave systems

on the planet.

These dark, twisting passageways

remain one of earth's final frontiers.

Few have been charted.

More people have walked on the moon than have witnessed

the farthest reaches of the Yucatan's

mysterious underworld.

Tunnels stretch into blackness,

and danger lies in every direction

of this subterranean maze.

A single wrong turn can mean the difference

between life and death.

There's no coming to the surface if you run out of air.

Hundreds of explorers have perished in the tunnels.

But daring divers were not the first to venture

into these unseen depths.

White catfish thrive in this lost, dark world.

They're night-time feeders, scouring the caverns

for bottom-dwelling invertebrates.

It's believed that as many as 3,000 cenotes