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In the heart of the Americas...
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..where continents collide...
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..there's a land full of natural riches...
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BIRDS HONK
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MONKEY GROWLS
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..a land of towering giants...
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..scorching sands...
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..and secret rivers.
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Where great civilisations rose...
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..and fell.
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To succeed here takes passion and spirit.
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This is a country rich in colour and culture...
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..a festival of life.
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This is...
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In Mexico's far south there's a forest full of secrets.
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It's March, the peak of the dry season.
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This pool is fast disappearing...
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..and it draws all the animals from the forest.
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Even the most elusive.
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For a young Morelet crocodile, the pool is also a hunting ground.
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There's plenty of potential prey.
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Trouble is, most are far too big.
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Finally, a realistic target.
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Even at his size, sneaking up in such shallow water isn't easy.
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He needs to change his technique.
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Perhaps an ambush will work.
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Almost.
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Success at last.
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But he's not the only crocodile here.
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And his taste of victory is short-lived.
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There's a reason water is so scarce in this corner of Mexico.
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This great forest, which stretches for almost 50,000 square miles,
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has no major rivers.
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And that's down to the region's unique geology.
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The forest stands upon a vast peninsula, the Yucatan.
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The entire peninsula is a gigantic slab of limestone.
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Limestone is incredibly porous.
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Any surface water swiftly drains away underground.
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And this shapes the lives of all who live here.
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For seven months of the year,
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virtually no rain falls,
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and by April, much of the forest is barren.
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For a mother spider monkey this can be challenging.
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She needs to produce enough milk to feed her baby.
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And that requires plenty of fresh fruit.
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Sometimes, that means travelling over two miles each day.
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But with long, slender fingers and no thumbs to get in the way
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this is child's play for an animal
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designed for life in the tree tops.
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She's taking her youngster to a special place...
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..where she knows there's fruit all year round.
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The ancient city of Calakmul,
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once one of the most powerful in the Maya civilisation.
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In the seventh century, this city was home to 50,000 people.
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The people of Calakmul
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cultivated trees that would fruit throughout the year.
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Centuries later, these ancient gardens have left a lasting legacy.
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There's more fruit here than anywhere else in the forest...
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..even at the height of the dry season.
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Over countless generations,
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the monkeys have passed this knowledge on to their youngsters.
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MONKEY CALLS
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At its peak in the eighth century,
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the Maya civilisation grew to almost 13 million people.
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Theirs was a highly sophisticated culture, advanced in mathematics,
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language and astronomy.
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The huge limestone temples, built to worship their gods,
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are a lasting testament to great feats of engineering.
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Almost every other ancient civilisation in history
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has been built beside a major river.
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So how did the Maya manage without one?
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There are almost 1.5 million direct descendants
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of the ancient Maya living in Mexico today.
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Don Roque and his wife Dona Su live on an isolated farm
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in the heart of the Yucatan's forest.
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GOAT BLEATS
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This farm has been in Don Roque's family for generations.
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With his children grown up and left home,
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nowadays, it's just him and Dona Su.
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They live off the land in a way
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little changed since the times of their ancestors.
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But there's more to their farm than meets the eye...
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..a huge natural well, known in the Yucatan as a cenote.
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A cenote is a collapsed cave.
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Over thousands of years, rainwater has eroded
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the soft limestone on the surface.
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Eventually, the roof weakened and fell.
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This cenote provides Don Roque with water all year round.
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But it's far from unique.
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It's one of several thousand cenotes scattered across the peninsula.
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Cenote comes from the Maya word ts'onot,
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meaning holes with water.
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It was these mysterious pools
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that enabled the Maya civilisation
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to flourish without a major river.
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And the greatest ancient cities were founded beside them.
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Each cenote is an oasis.
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The surrounding forest remains lush,
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even at the height of the dry season.
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Filtered through the limestone, the water is full of minerals.
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It supports a unique community of plants and animals.
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Some fish in the Yucatan cenotes are found nowhere else on Earth.
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Within the dense forest,
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a cenote can remain hidden, even when close by.
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But the Maya had an almost sure-fire way of finding them...
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BIRD CALLS
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..the distinctive call of the turquoise-browed motmot.
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Virtually every cenote has its own colony.
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Elsewhere, motmots prefer to nest alone,
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digging their burrows along river banks.
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MOTMOT CALLS
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Here in the Yucatan, they're forced together,
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sharing limited space on the cenote's soft limestone walls.
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Before a male can start looking for a mate,
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he needs to secure a nest site.
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But this cenote is already crowded.
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17 other pairs are nesting here.
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If he's to fit in, this new arrival must use the right body language.
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MOTMOT CALLS
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A raised turquoise brow is intended to intimidate rivals.
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The pendulous tick-tock of their tails
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has earned motmots the nickname clock birds.
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This male must persevere if he wants to breed this year.
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At last, he's earned his own little ledge of limestone.
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With a gift of food,
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he can get down to the business of attracting a mate.
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There are so many cenotes in the Yucatan
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because of a dramatic event
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that changed the history of life on Earth.
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Around 65 million years ago,
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a huge meteorite collided with what is now the Yucatan's north coast.
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Its effect was so catastrophic...
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..it's believed to have wiped out the dinosaurs.
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In an instant, the impact fractured the limestone platform,
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creating what has become known as the ring of cenotes.
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Satellites have revealed over 200 cluster around its crater.
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Satellite technology is also helping modern-day explorers in the Yucatan.
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And new cenotes are still being discovered today.
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It should be about 50 metres that way. You see anything?
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Sam Meacham and Fred Devos are part of an international community
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of cave divers that has been exploring the Yucatan
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since the 1980s.
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I started out for the thrill of being able to explore something.
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And from a young age, I just was imbued
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with the desire to go out and explore this world.
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Each new cenote adds another piece to the puzzle.
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- Oh, look at this, Sam.
- Wow.
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The moment of discovery, right? You can't beat it.
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In the last 30 years, cave divers in the Yucatan
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have revealed something extraordinary.
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The cenotes are not isolated wells.
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Each is connected by an intricate network of caves and passageways,
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spreading like a spider's web across the peninsula.
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Divers have discovered over 350 caves
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and nearly 1,000 miles of flooded tunnels.
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Including the two longest underwater cave systems on the planet.
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Many of these caverns have seen fewer visitors than the moon.
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The great majority remains uncharted.
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For the ancient Maya, the labyrinth of caves
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was out of reach but not beyond imagination.
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This secret underworld was held sacred.
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Home to the 12 gods of Xibalba,
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it was a place both feared and revered.
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But the ancient Maya recognised their link to this unknown world.
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Trees.
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Always plumbed into the water below,
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trees allow life above ground to flourish.
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No tree is better equipped to do this than the strangler fig.
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Its roots can reach 30 metres into the water below.
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Don Roque uses the fig's roots to enter the underworld...
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..navigating his way down steps carved by his grandfather
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over half a century ago.
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At the bottom of the cenote, Don Roque has a secret garden.
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Here he cultivates a plant that wouldn't survive
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elsewhere on his farm.
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Coffee.
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Above ground it's too hot and dry for coffee to grow.
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Down here it's two degrees cooler and a lot more humid.
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Don Roque's cenote is also a haven for insects.
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Food for a colony of resident cave swallows.
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By May the swallows are intent on one thing.
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Breeding.
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The cenote provides everything they need.
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Down here they have a ready supply of soft mud.
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They use this to craft their nests
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together with fibres plucked from the hanging fig roots.
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All being well, in around a month's time,
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they'll each have a brood of up to five chicks.
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Don Roque's cenote has its own community of life
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thanks to the constant presence of water.
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But the rest of his farm requires something more.
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He plants his land with a mixture of corn, beans and squash
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in a tradition passed down the generations.
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A mix of crops helps maintain the fragile soil
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which lies in a thin layer on top of the limestone.
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But he can't draw enough water from his cenote
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to irrigate an entire field.
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For his crops to grow, Don Roque needs it to rain.
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By late May, change is in the air.
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The Yucatan relies on weather systems
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that develop thousands of miles away.
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Gathering ever more water as they cross the Atlantic,
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they bring the seasonal rains.
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The ancient Maya believed the rain was a gift from the gods.
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For Don Roque, the old gods are very much alive.
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RUMBLE OF THUNDER
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The most important of all is Chaac, the life-giving god of rain.
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It is Chaac who determines each thundercloud.
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CRACK OF LIGHTNING
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RUMBLE OF THUNDER
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Finally letting the rain fall.
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1,200 years ago, something happened that sent the Maya civilisation
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into a dramatic decline.
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Chaac withheld the seasonal rains.
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The Yucatan was gripped by a series of devastating droughts.
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Over 80% of the population vanished.
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One by one, the bustling cities were abandoned
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and reclaimed by the forest.
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MONKEYS HOWLING
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This year the gods have been kind.
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By July, it's raining almost every day.
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MONKEYS GRUNT AND HOWL
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This brings new growth to the forest.
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Good news for the Yucatan's noisiest residents.
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LOUD HOWL
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Howler monkeys.
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Their calls carry for over a mile.
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MONKEYS HOWL
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They howl to let neighbouring groups know their whereabouts.
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This patch of the forest is worth protecting.
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The troop has found a tree with young, succulent leaves -
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a particular favourite.
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But a diet of leaves takes time to digest.
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So howlers spend up to 80% of their time resting...
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..making them amongst the most peaceful of all monkeys
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despite their fearsome calls.
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Baby howlers feed on their mother's milk until almost a year old.
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This is much easier to digest.
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With his mother trying to sleep off her lunch,
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nap time can be a bit dull for a baby.
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RUMBLE OF THUNDER
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All life in the Yucatan benefits from the rain.
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Trees burst into fruit.
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All the new plant growth
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provides food for an explosion of insect life.
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There are over 3,000 different species on the peninsula.
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The insects become food for others.
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Almost all the birds in the forest schedule their nesting to coincide
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with this seasonal bounty.
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The regular rain is also good news for Don Roque.
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In just six weeks his corn has shot up.
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But the explosion of plant-eating insects
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could cause Don Roque problems.
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For his crops to thrive, he needs the help of a natural ally.
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In the heart of the Yucatan,
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there's a cave the locals call El Volcan de los Murcielagos.
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The bat volcano.
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Every day, an hour before dusk,
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it erupts.
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More than three million bats exit the cave.
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The ancient Maya associated bats with death.
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This was a swarm straight from the underworld.
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But bats are vital to life in the Yucatan,
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keeping its insect population in check.
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Each can eat its own body weight in insects every night.
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While the bats are focused on the insects,
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others are watching them.
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Grey hawks...
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..and bat falcons.
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These birds are skilled aerial predators,
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their talons perfectly designed to snatch the bats from the air.
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There are other more surprising hunters here.
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Brown jays.
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They normally eat insects and fruit.
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But the brown jays here have developed a taste for bats.
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The jays don't use talons to catch their prey.
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They use their beaks.
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The loss of ten or so bats barely makes a dent on the swarm.
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Tonight, the colony will consume around 30 tonnes of insects.
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A vital service that keeps the entire forest healthy.
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It's late September...
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..and it has rained almost every day for the last three months.
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Don Roque's corn is now fully ripe.
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In a tradition dating back to the times of their ancient ancestors,
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the first corn of the harvest serves a special purpose.
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Dona Su crushes it to make a sacred drink called atole.
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This is offered to the gods in a ceremony of thanksgiving.
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SPEAKS IN NATIVE LANGUAGE
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From June to November, the Yucatan receives 100 centimetres of rain.
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Across the peninsula, that's over 36 trillion gallons.
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The cenotes are refilled.
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But most of this water seeps through the limestone into the underworld.
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The work of cave divers like Sam is furthering our understanding
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of the Yucatan's unique water system.
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'We're still exploring here.
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'And the more we explore, the more questions we ask.
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'Selfishly, I hope we finish in my lifetime.
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'I don't think we will.'
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Metre by metre, the divers are mapping the underground labyrinth.
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It's a painstaking process with every measurement taken by hand.
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But this has revealed something remarkable.
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The water in the caves isn't just locked within the limestone.
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It flows.
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The Yucatan's missing rivers are underground.
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Like all rivers, they are compelled towards the coast.
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Here, the fresh water meets salt water brought by the incoming tide.
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This border is called the halocline.
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Containing more dissolved particles,
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the salt water is heavier and lies below the fresh water...
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..giving the illusion of a surface.
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But after months of heavy rain...
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..the flow of fresh water prevails.
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At the coast, it leaves the underworld behind.
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00:45:30,400 --> 00:45:34,840
Mangroves flourish in the mixture of salt and fresh water.
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The roots provide ideal nurseries for schools of fish.
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And gentle giants are drawn to the Yucatan's shores.
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Manatees come to drink at the outpouring of fresh water.
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As the fresh water flows through the mangroves,
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the trees release tannins, staining it brown.
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Eventually the fresh water reaches its final destination.
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It pours out into the Gulf of Mexico in huge volumes.
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DRAMATIC MUSIC
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Replenished by billions of gallons of fresh water,
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these vast coastal lagoons are the ideal home
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for a colony of the Yucatan's most colourful residents.
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Caribbean flamingos.
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It's been a good year for the colony,
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with nearly 15,000 new additions.
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Finding your chick in such a big crowd isn't easy.
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FLAMINGOS SQUAWK
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But flamingos have incredibly good hearing
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00:49:10,520 --> 00:49:14,040
and her youngster has his own unique voice.
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00:49:20,760 --> 00:49:22,600
Just six weeks ago,
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the chick was little bigger than his mother's beak.
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These lagoons are full of brine shrimps.
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The beta-carotene in these tiny crustaceans
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gives the flamingos their distinctive colour.
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But it will be another two years before he starts to turn pink.
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Here on the Yucatan's coast, as elsewhere on the peninsula,
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all owe their survival to the secret underworld.
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Without its great hidden rivers,
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this would be a hot, dry and hostile place.
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00:51:07,520 --> 00:51:10,040
Born of a unique history,
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the underworld hidden beneath this vast forest
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gave rise to the ancient Maya civilisation...
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..and it remains the lifeblood of the Yucatan.
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Capturing the beauty of the Yucatan's underworld
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meant sending a film crew into one of the most alien environments
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on the planet.
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To do this safely required expertise.
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00:51:54,120 --> 00:51:58,000
Between them, this team has over 50 years' experience
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diving on the peninsula.
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I want to make sure we get that right, you know?
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00:52:03,520 --> 00:52:06,520
Cameraman Mike Madden was one of the pioneers
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of cave exploration here back in the 1980s.
356
00:52:09,880 --> 00:52:13,360
And Sam Meacham is part of a team currently mapping
357
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one of the region's two longest caves.
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There's a common misconception that cave divers are these
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adrenaline junkie risk-takers cheating death on every dive.
360
00:52:24,960 --> 00:52:28,040
Were tarantulas in our risk management form?
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'When, in fact, we're probably some of the most cautious people
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00:52:32,040 --> 00:52:33,720
'on the planet.'
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There's a very real need for caution.
364
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All diving carries risks.
365
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But cave diving heightens these risks.
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Surfacing isn't an option if you run out of air.
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00:53:03,040 --> 00:53:06,520
This particular cave system, known as Taj Mahal,
368
00:53:06,520 --> 00:53:09,040
was first explored in 1995.
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00:53:11,520 --> 00:53:15,040
It has over three miles of flooded passageways,
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some descending almost 30 metres.
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Filming here poses another challenge.
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00:53:22,280 --> 00:53:25,040
Most of the cave is pitch black.
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So the first thing the divers need to do
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is set up a highly specialised underwater lighting rig.
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Four 1,200-watt lights.
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00:53:42,200 --> 00:53:44,760
100 metres of cable.
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00:53:47,520 --> 00:53:51,040
All of it has to be carried into the cave by hand.
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00:53:57,520 --> 00:54:01,040
Lighting the caves on this scale is a new challenge for the team
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00:54:01,040 --> 00:54:03,520
and draws on all their expertise.
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00:54:06,680 --> 00:54:11,760
Since the 1980s, the dedicated work of divers like Sam and Mike
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00:54:11,760 --> 00:54:16,240
has helped put nearly 1,000 miles of the Yucatan's underworld on the map.
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00:54:17,240 --> 00:54:20,400
But the vast majority remains uncharted.
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00:54:21,400 --> 00:54:26,040
It's pretty amazing that 35 years after the beginning
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00:54:26,040 --> 00:54:29,040
of cave diving exploration in this area,
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00:54:29,040 --> 00:54:32,760
I would say we're still really beginning to understand
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00:54:32,760 --> 00:54:34,760
what's going on here.
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And now there's an added urgency to their work.
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00:54:43,280 --> 00:54:48,040
Over the last 50 years the Yucatan's population has skyrocketed.
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The city of Cancun is one of the fastest-growing tourist destinations
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in the world, with over five million visitors every year.
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00:55:07,040 --> 00:55:10,280
This creates jobs, but the demand for fresh water
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00:55:10,280 --> 00:55:13,760
is putting pressure on the peninsula's underground rivers.
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00:55:14,760 --> 00:55:18,280
The future of this area is dependent on tourism
394
00:55:18,280 --> 00:55:23,520
and so how do we sustainably manage these incredible natural resources
395
00:55:23,520 --> 00:55:27,520
so that Mexico can continue to receive
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00:55:27,520 --> 00:55:31,040
10-12% of its gross domestic product through tourism
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on a 100-mile strip of beach?
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That's important to the future of Mexico,
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not only of this region but the country as a whole.
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00:55:44,680 --> 00:55:48,200
Maps of the underground river systems are far from complete.
401
00:55:49,560 --> 00:55:53,040
New construction work may be taking place above caves
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00:55:53,040 --> 00:55:55,280
that are yet to be discovered.
403
00:55:58,560 --> 00:56:05,040
The entire city of Cancun draws its drinking water from 142 cenotes,
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00:56:05,040 --> 00:56:08,040
some of which are now at risk of pollution.
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00:56:15,040 --> 00:56:18,280
It's a race against time to reveal the importance
406
00:56:18,280 --> 00:56:20,760
of the Yucatan's hidden underworld.
407
00:56:27,040 --> 00:56:30,280
Back at Taj Mahal, with the lights in place,
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00:56:30,280 --> 00:56:32,760
the crew are ready to begin filming.
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All our focus is going to be this direction today.
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00:56:38,120 --> 00:56:42,200
Every image they film must be meticulously planned.
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Get over. More light.
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00:56:49,520 --> 00:56:51,560
Less light.
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Once the dive begins,
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all communication is through hand signals.
415
00:57:10,000 --> 00:57:14,520
At last the team can capture the splendour of the hidden underworld.
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For most of us, this flooded labyrinth
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00:57:24,760 --> 00:57:29,040
remains just as inaccessible as it was to the ancient Maya.
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00:57:31,040 --> 00:57:36,280
But for the cave divers devoted to exploring this world,
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00:57:36,280 --> 00:57:41,040
the hope is that by opening our eyes to its beauty and importance,
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they will help safeguard its future.
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00:58:02,080 --> 00:58:07,240
In the next episode, we head to Mexico's remotest region.
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00:58:07,240 --> 00:58:09,120
The far north.
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00:58:09,120 --> 00:58:13,240
A land of great deserts and rich prairies.
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00:58:15,520 --> 00:58:18,320
Home to the toughest of characters...
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00:58:19,320 --> 00:58:21,640
..secret worlds...
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00:58:22,640 --> 00:58:26,040
..and some of Mexico's rarest wildlife.
33191
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