0:00:05 > 0:00:06In the heart of the Americas...
0:00:08 > 0:00:09..where continents collide...
0:00:11 > 0:00:14..there's a land full of natural riches...
0:00:20 > 0:00:22BIRDS HONK
0:00:26 > 0:00:27MONKEY GROWLS
0:00:34 > 0:00:36..a land of towering giants...
0:00:39 > 0:00:41..scorching sands...
0:00:44 > 0:00:46..and secret rivers.
0:00:53 > 0:00:56Where great civilisations rose...
0:00:57 > 0:00:58..and fell.
0:01:03 > 0:01:06To succeed here takes passion and spirit.
0:01:12 > 0:01:15This is a country rich in colour and culture...
0:01:20 > 0:01:22..a festival of life.
0:01:35 > 0:01:38This is...
0:02:07 > 0:02:11In Mexico's far south there's a forest full of secrets.
0:02:16 > 0:02:20It's March, the peak of the dry season.
0:02:22 > 0:02:24This pool is fast disappearing...
0:02:26 > 0:02:28..and it draws all the animals from the forest.
0:02:41 > 0:02:43Even the most elusive.
0:03:10 > 0:03:15For a young Morelet crocodile, the pool is also a hunting ground.
0:03:26 > 0:03:28There's plenty of potential prey.
0:03:30 > 0:03:33Trouble is, most are far too big.
0:03:48 > 0:03:50Finally, a realistic target.
0:04:11 > 0:04:17Even at his size, sneaking up in such shallow water isn't easy.
0:04:23 > 0:04:24He needs to change his technique.
0:04:30 > 0:04:32Perhaps an ambush will work.
0:04:52 > 0:04:53Almost.
0:05:22 > 0:05:23Success at last.
0:05:26 > 0:05:29But he's not the only crocodile here.
0:05:45 > 0:05:48And his taste of victory is short-lived.
0:05:55 > 0:05:58There's a reason water is so scarce in this corner of Mexico.
0:06:04 > 0:06:09This great forest, which stretches for almost 50,000 square miles,
0:06:09 > 0:06:10has no major rivers.
0:06:15 > 0:06:18And that's down to the region's unique geology.
0:06:23 > 0:06:27The forest stands upon a vast peninsula, the Yucatan.
0:06:34 > 0:06:37The entire peninsula is a gigantic slab of limestone.
0:06:41 > 0:06:43Limestone is incredibly porous.
0:06:46 > 0:06:50Any surface water swiftly drains away underground.
0:06:52 > 0:06:54And this shapes the lives of all who live here.
0:07:13 > 0:07:15For seven months of the year,
0:07:15 > 0:07:18virtually no rain falls,
0:07:18 > 0:07:21and by April, much of the forest is barren.
0:07:23 > 0:07:26For a mother spider monkey this can be challenging.
0:07:29 > 0:07:32She needs to produce enough milk to feed her baby.
0:07:39 > 0:07:42And that requires plenty of fresh fruit.
0:07:48 > 0:07:53Sometimes, that means travelling over two miles each day.
0:07:57 > 0:08:01But with long, slender fingers and no thumbs to get in the way
0:08:01 > 0:08:04this is child's play for an animal
0:08:04 > 0:08:06designed for life in the tree tops.
0:08:25 > 0:08:28She's taking her youngster to a special place...
0:08:29 > 0:08:33..where she knows there's fruit all year round.
0:08:49 > 0:08:52The ancient city of Calakmul,
0:08:52 > 0:08:56once one of the most powerful in the Maya civilisation.
0:09:01 > 0:09:05In the seventh century, this city was home to 50,000 people.
0:09:13 > 0:09:15The people of Calakmul
0:09:15 > 0:09:18cultivated trees that would fruit throughout the year.
0:09:24 > 0:09:29Centuries later, these ancient gardens have left a lasting legacy.
0:09:41 > 0:09:45There's more fruit here than anywhere else in the forest...
0:09:46 > 0:09:48..even at the height of the dry season.
0:09:52 > 0:09:54Over countless generations,
0:09:54 > 0:09:57the monkeys have passed this knowledge on to their youngsters.
0:10:04 > 0:10:05MONKEY CALLS
0:10:36 > 0:10:39At its peak in the eighth century,
0:10:39 > 0:10:43the Maya civilisation grew to almost 13 million people.
0:10:51 > 0:10:57Theirs was a highly sophisticated culture, advanced in mathematics,
0:10:57 > 0:11:00language and astronomy.
0:11:03 > 0:11:07The huge limestone temples, built to worship their gods,
0:11:07 > 0:11:11are a lasting testament to great feats of engineering.
0:11:17 > 0:11:21Almost every other ancient civilisation in history
0:11:21 > 0:11:23has been built beside a major river.
0:11:25 > 0:11:28So how did the Maya manage without one?
0:11:31 > 0:11:35There are almost 1.5 million direct descendants
0:11:35 > 0:11:37of the ancient Maya living in Mexico today.
0:11:40 > 0:11:44Don Roque and his wife Dona Su live on an isolated farm
0:11:44 > 0:11:47in the heart of the Yucatan's forest.
0:11:50 > 0:11:53GOAT BLEATS
0:12:06 > 0:12:09This farm has been in Don Roque's family for generations.
0:12:13 > 0:12:16With his children grown up and left home,
0:12:16 > 0:12:18nowadays, it's just him and Dona Su.
0:12:24 > 0:12:27They live off the land in a way
0:12:27 > 0:12:30little changed since the times of their ancestors.
0:12:40 > 0:12:43But there's more to their farm than meets the eye...
0:13:03 > 0:13:08..a huge natural well, known in the Yucatan as a cenote.
0:13:19 > 0:13:21A cenote is a collapsed cave.
0:13:24 > 0:13:27Over thousands of years, rainwater has eroded
0:13:27 > 0:13:31the soft limestone on the surface.
0:13:31 > 0:13:34Eventually, the roof weakened and fell.
0:13:42 > 0:13:46This cenote provides Don Roque with water all year round.
0:13:55 > 0:13:57But it's far from unique.
0:14:03 > 0:14:08It's one of several thousand cenotes scattered across the peninsula.
0:14:13 > 0:14:17Cenote comes from the Maya word ts'onot,
0:14:17 > 0:14:19meaning holes with water.
0:14:26 > 0:14:28It was these mysterious pools
0:14:28 > 0:14:30that enabled the Maya civilisation
0:14:30 > 0:14:33to flourish without a major river.
0:14:39 > 0:14:43And the greatest ancient cities were founded beside them.
0:14:50 > 0:14:52Each cenote is an oasis.
0:14:57 > 0:15:00The surrounding forest remains lush,
0:15:00 > 0:15:02even at the height of the dry season.
0:15:08 > 0:15:12Filtered through the limestone, the water is full of minerals.
0:15:15 > 0:15:19It supports a unique community of plants and animals.
0:15:22 > 0:15:27Some fish in the Yucatan cenotes are found nowhere else on Earth.
0:15:35 > 0:15:37Within the dense forest,
0:15:37 > 0:15:40a cenote can remain hidden, even when close by.
0:15:44 > 0:15:47But the Maya had an almost sure-fire way of finding them...
0:15:50 > 0:15:51BIRD CALLS
0:15:53 > 0:15:56..the distinctive call of the turquoise-browed motmot.
0:16:03 > 0:16:06Virtually every cenote has its own colony.
0:16:09 > 0:16:12Elsewhere, motmots prefer to nest alone,
0:16:12 > 0:16:15digging their burrows along river banks.
0:16:16 > 0:16:18MOTMOT CALLS
0:16:19 > 0:16:22Here in the Yucatan, they're forced together,
0:16:22 > 0:16:26sharing limited space on the cenote's soft limestone walls.
0:16:31 > 0:16:34Before a male can start looking for a mate,
0:16:34 > 0:16:36he needs to secure a nest site.
0:16:38 > 0:16:40But this cenote is already crowded.
0:16:40 > 0:16:4317 other pairs are nesting here.
0:16:49 > 0:16:54If he's to fit in, this new arrival must use the right body language.
0:16:57 > 0:16:58MOTMOT CALLS
0:17:07 > 0:17:11A raised turquoise brow is intended to intimidate rivals.
0:17:29 > 0:17:31The pendulous tick-tock of their tails
0:17:31 > 0:17:35has earned motmots the nickname clock birds.
0:17:41 > 0:17:45This male must persevere if he wants to breed this year.
0:17:53 > 0:17:57At last, he's earned his own little ledge of limestone.
0:18:03 > 0:18:06With a gift of food,
0:18:06 > 0:18:09he can get down to the business of attracting a mate.
0:18:19 > 0:18:22There are so many cenotes in the Yucatan
0:18:22 > 0:18:24because of a dramatic event
0:18:24 > 0:18:26that changed the history of life on Earth.
0:18:39 > 0:18:41Around 65 million years ago,
0:18:41 > 0:18:46a huge meteorite collided with what is now the Yucatan's north coast.
0:18:49 > 0:18:52Its effect was so catastrophic...
0:18:53 > 0:18:55..it's believed to have wiped out the dinosaurs.
0:18:57 > 0:19:02In an instant, the impact fractured the limestone platform,
0:19:02 > 0:19:05creating what has become known as the ring of cenotes.
0:19:07 > 0:19:11Satellites have revealed over 200 cluster around its crater.
0:19:15 > 0:19:20Satellite technology is also helping modern-day explorers in the Yucatan.
0:19:22 > 0:19:26And new cenotes are still being discovered today.
0:19:26 > 0:19:30It should be about 50 metres that way. You see anything?
0:19:30 > 0:19:35Sam Meacham and Fred Devos are part of an international community
0:19:35 > 0:19:38of cave divers that has been exploring the Yucatan
0:19:38 > 0:19:40since the 1980s.
0:19:42 > 0:19:46I started out for the thrill of being able to explore something.
0:19:46 > 0:19:49And from a young age, I just was imbued
0:19:49 > 0:19:51with the desire to go out and explore this world.
0:19:53 > 0:19:56Each new cenote adds another piece to the puzzle.
0:19:59 > 0:20:02- Oh, look at this, Sam.- Wow.
0:20:02 > 0:20:05The moment of discovery, right? You can't beat it.
0:20:17 > 0:20:21In the last 30 years, cave divers in the Yucatan
0:20:21 > 0:20:23have revealed something extraordinary.
0:20:27 > 0:20:30The cenotes are not isolated wells.
0:20:35 > 0:20:39Each is connected by an intricate network of caves and passageways,
0:20:39 > 0:20:43spreading like a spider's web across the peninsula.
0:20:50 > 0:20:56Divers have discovered over 350 caves
0:20:56 > 0:21:00and nearly 1,000 miles of flooded tunnels.
0:21:01 > 0:21:06Including the two longest underwater cave systems on the planet.
0:21:37 > 0:21:42Many of these caverns have seen fewer visitors than the moon.
0:21:45 > 0:21:48The great majority remains uncharted.
0:22:10 > 0:22:13For the ancient Maya, the labyrinth of caves
0:22:13 > 0:22:17was out of reach but not beyond imagination.
0:22:18 > 0:22:22This secret underworld was held sacred.
0:22:23 > 0:22:26Home to the 12 gods of Xibalba,
0:22:26 > 0:22:29it was a place both feared and revered.
0:22:34 > 0:22:38But the ancient Maya recognised their link to this unknown world.
0:22:39 > 0:22:41Trees.
0:22:48 > 0:22:51Always plumbed into the water below,
0:22:51 > 0:22:54trees allow life above ground to flourish.
0:22:58 > 0:23:03No tree is better equipped to do this than the strangler fig.
0:23:04 > 0:23:07Its roots can reach 30 metres into the water below.
0:23:26 > 0:23:31Don Roque uses the fig's roots to enter the underworld...
0:23:32 > 0:23:36..navigating his way down steps carved by his grandfather
0:23:36 > 0:23:39over half a century ago.
0:24:07 > 0:24:11At the bottom of the cenote, Don Roque has a secret garden.
0:24:17 > 0:24:21Here he cultivates a plant that wouldn't survive
0:24:21 > 0:24:23elsewhere on his farm.
0:24:23 > 0:24:25Coffee.
0:24:30 > 0:24:34Above ground it's too hot and dry for coffee to grow.
0:24:36 > 0:24:40Down here it's two degrees cooler and a lot more humid.
0:24:45 > 0:24:49Don Roque's cenote is also a haven for insects.
0:24:49 > 0:24:52Food for a colony of resident cave swallows.
0:25:11 > 0:25:15By May the swallows are intent on one thing.
0:25:17 > 0:25:18Breeding.
0:25:25 > 0:25:28The cenote provides everything they need.
0:25:35 > 0:25:39Down here they have a ready supply of soft mud.
0:25:43 > 0:25:46They use this to craft their nests
0:25:46 > 0:25:51together with fibres plucked from the hanging fig roots.
0:26:06 > 0:26:10All being well, in around a month's time,
0:26:10 > 0:26:13they'll each have a brood of up to five chicks.
0:26:19 > 0:26:23Don Roque's cenote has its own community of life
0:26:23 > 0:26:26thanks to the constant presence of water.
0:26:26 > 0:26:29But the rest of his farm requires something more.
0:26:30 > 0:26:34He plants his land with a mixture of corn, beans and squash
0:26:34 > 0:26:37in a tradition passed down the generations.
0:26:41 > 0:26:44A mix of crops helps maintain the fragile soil
0:26:44 > 0:26:47which lies in a thin layer on top of the limestone.
0:26:50 > 0:26:53But he can't draw enough water from his cenote
0:26:53 > 0:26:56to irrigate an entire field.
0:26:58 > 0:27:02For his crops to grow, Don Roque needs it to rain.
0:27:06 > 0:27:09By late May, change is in the air.
0:27:14 > 0:27:17The Yucatan relies on weather systems
0:27:17 > 0:27:19that develop thousands of miles away.
0:27:23 > 0:27:27Gathering ever more water as they cross the Atlantic,
0:27:27 > 0:27:29they bring the seasonal rains.
0:27:35 > 0:27:40The ancient Maya believed the rain was a gift from the gods.
0:27:44 > 0:27:49For Don Roque, the old gods are very much alive.
0:27:49 > 0:27:51RUMBLE OF THUNDER
0:27:58 > 0:28:04The most important of all is Chaac, the life-giving god of rain.
0:28:29 > 0:28:33It is Chaac who determines each thundercloud.
0:28:33 > 0:28:35CRACK OF LIGHTNING
0:28:40 > 0:28:42RUMBLE OF THUNDER
0:28:46 > 0:28:49Finally letting the rain fall.
0:28:57 > 0:29:021,200 years ago, something happened that sent the Maya civilisation
0:29:02 > 0:29:04into a dramatic decline.
0:29:05 > 0:29:08Chaac withheld the seasonal rains.
0:29:11 > 0:29:16The Yucatan was gripped by a series of devastating droughts.
0:29:22 > 0:29:24Over 80% of the population vanished.
0:29:33 > 0:29:37One by one, the bustling cities were abandoned
0:29:37 > 0:29:40and reclaimed by the forest.
0:29:45 > 0:29:47MONKEYS HOWLING
0:29:47 > 0:29:50This year the gods have been kind.
0:29:50 > 0:29:54By July, it's raining almost every day.
0:29:55 > 0:29:58MONKEYS GRUNT AND HOWL
0:30:00 > 0:30:04This brings new growth to the forest.
0:30:04 > 0:30:08Good news for the Yucatan's noisiest residents.
0:30:09 > 0:30:11LOUD HOWL
0:30:17 > 0:30:19Howler monkeys.
0:30:23 > 0:30:25Their calls carry for over a mile.
0:30:26 > 0:30:28MONKEYS HOWL
0:30:30 > 0:30:34They howl to let neighbouring groups know their whereabouts.
0:30:44 > 0:30:47This patch of the forest is worth protecting.
0:30:49 > 0:30:53The troop has found a tree with young, succulent leaves -
0:30:53 > 0:30:55a particular favourite.
0:31:05 > 0:31:09But a diet of leaves takes time to digest.
0:31:14 > 0:31:19So howlers spend up to 80% of their time resting...
0:31:20 > 0:31:23..making them amongst the most peaceful of all monkeys
0:31:23 > 0:31:26despite their fearsome calls.
0:31:30 > 0:31:35Baby howlers feed on their mother's milk until almost a year old.
0:31:36 > 0:31:38This is much easier to digest.
0:31:53 > 0:31:57With his mother trying to sleep off her lunch,
0:31:57 > 0:32:01nap time can be a bit dull for a baby.
0:32:16 > 0:32:18RUMBLE OF THUNDER
0:32:24 > 0:32:27All life in the Yucatan benefits from the rain.
0:32:38 > 0:32:40Trees burst into fruit.
0:32:46 > 0:32:49All the new plant growth
0:32:49 > 0:32:52provides food for an explosion of insect life.
0:32:53 > 0:32:57There are over 3,000 different species on the peninsula.
0:33:03 > 0:33:06The insects become food for others.
0:33:13 > 0:33:18Almost all the birds in the forest schedule their nesting to coincide
0:33:18 > 0:33:20with this seasonal bounty.
0:33:39 > 0:33:43The regular rain is also good news for Don Roque.
0:33:46 > 0:33:49In just six weeks his corn has shot up.
0:34:12 > 0:34:15But the explosion of plant-eating insects
0:34:15 > 0:34:17could cause Don Roque problems.
0:34:20 > 0:34:25For his crops to thrive, he needs the help of a natural ally.
0:34:39 > 0:34:41In the heart of the Yucatan,
0:34:41 > 0:34:47there's a cave the locals call El Volcan de los Murcielagos.
0:34:49 > 0:34:51The bat volcano.
0:35:01 > 0:35:04Every day, an hour before dusk,
0:35:04 > 0:35:06it erupts.
0:35:23 > 0:35:28More than three million bats exit the cave.
0:36:14 > 0:36:17The ancient Maya associated bats with death.
0:36:21 > 0:36:25This was a swarm straight from the underworld.
0:36:31 > 0:36:34But bats are vital to life in the Yucatan,
0:36:34 > 0:36:37keeping its insect population in check.
0:36:39 > 0:36:43Each can eat its own body weight in insects every night.
0:36:51 > 0:36:54While the bats are focused on the insects,
0:36:54 > 0:36:56others are watching them.
0:36:58 > 0:37:01Grey hawks...
0:37:02 > 0:37:05..and bat falcons.
0:37:07 > 0:37:11These birds are skilled aerial predators,
0:37:11 > 0:37:15their talons perfectly designed to snatch the bats from the air.
0:37:35 > 0:37:39There are other more surprising hunters here.
0:37:42 > 0:37:44Brown jays.
0:37:51 > 0:37:54They normally eat insects and fruit.
0:38:05 > 0:38:10But the brown jays here have developed a taste for bats.
0:38:22 > 0:38:25The jays don't use talons to catch their prey.
0:38:28 > 0:38:30They use their beaks.
0:38:59 > 0:39:03The loss of ten or so bats barely makes a dent on the swarm.
0:39:14 > 0:39:19Tonight, the colony will consume around 30 tonnes of insects.
0:39:19 > 0:39:24A vital service that keeps the entire forest healthy.
0:39:51 > 0:39:53It's late September...
0:39:54 > 0:39:59..and it has rained almost every day for the last three months.
0:40:00 > 0:40:03Don Roque's corn is now fully ripe.
0:40:51 > 0:40:56In a tradition dating back to the times of their ancient ancestors,
0:40:56 > 0:41:00the first corn of the harvest serves a special purpose.
0:41:06 > 0:41:11Dona Su crushes it to make a sacred drink called atole.
0:41:17 > 0:41:22This is offered to the gods in a ceremony of thanksgiving.
0:41:34 > 0:41:37SPEAKS IN NATIVE LANGUAGE
0:42:18 > 0:42:23From June to November, the Yucatan receives 100 centimetres of rain.
0:42:28 > 0:42:33Across the peninsula, that's over 36 trillion gallons.
0:42:42 > 0:42:44The cenotes are refilled.
0:42:51 > 0:42:56But most of this water seeps through the limestone into the underworld.
0:43:00 > 0:43:04The work of cave divers like Sam is furthering our understanding
0:43:04 > 0:43:07of the Yucatan's unique water system.
0:43:07 > 0:43:09'We're still exploring here.
0:43:09 > 0:43:13'And the more we explore, the more questions we ask.
0:43:14 > 0:43:16'Selfishly, I hope we finish in my lifetime.
0:43:16 > 0:43:18'I don't think we will.'
0:43:26 > 0:43:31Metre by metre, the divers are mapping the underground labyrinth.
0:43:33 > 0:43:38It's a painstaking process with every measurement taken by hand.
0:43:41 > 0:43:45But this has revealed something remarkable.
0:43:51 > 0:43:55The water in the caves isn't just locked within the limestone.
0:43:57 > 0:43:59It flows.
0:44:03 > 0:44:06The Yucatan's missing rivers are underground.
0:44:25 > 0:44:29Like all rivers, they are compelled towards the coast.
0:44:35 > 0:44:41Here, the fresh water meets salt water brought by the incoming tide.
0:44:49 > 0:44:52This border is called the halocline.
0:44:56 > 0:44:58Containing more dissolved particles,
0:44:58 > 0:45:03the salt water is heavier and lies below the fresh water...
0:45:05 > 0:45:08..giving the illusion of a surface.
0:45:12 > 0:45:14But after months of heavy rain...
0:45:15 > 0:45:18..the flow of fresh water prevails.
0:45:21 > 0:45:25At the coast, it leaves the underworld behind.
0:45:30 > 0:45:34Mangroves flourish in the mixture of salt and fresh water.
0:45:37 > 0:45:41The roots provide ideal nurseries for schools of fish.
0:45:45 > 0:45:49And gentle giants are drawn to the Yucatan's shores.
0:45:56 > 0:46:01Manatees come to drink at the outpouring of fresh water.
0:46:21 > 0:46:25As the fresh water flows through the mangroves,
0:46:25 > 0:46:28the trees release tannins, staining it brown.
0:46:49 > 0:46:53Eventually the fresh water reaches its final destination.
0:47:07 > 0:47:12It pours out into the Gulf of Mexico in huge volumes.
0:47:13 > 0:47:16DRAMATIC MUSIC
0:47:51 > 0:47:55Replenished by billions of gallons of fresh water,
0:47:55 > 0:47:58these vast coastal lagoons are the ideal home
0:47:58 > 0:48:02for a colony of the Yucatan's most colourful residents.
0:48:05 > 0:48:07Caribbean flamingos.
0:48:45 > 0:48:48It's been a good year for the colony,
0:48:48 > 0:48:51with nearly 15,000 new additions.
0:49:01 > 0:49:05Finding your chick in such a big crowd isn't easy.
0:49:05 > 0:49:07FLAMINGOS SQUAWK
0:49:07 > 0:49:10But flamingos have incredibly good hearing
0:49:10 > 0:49:14and her youngster has his own unique voice.
0:49:20 > 0:49:22Just six weeks ago,
0:49:22 > 0:49:25the chick was little bigger than his mother's beak.
0:49:44 > 0:49:47These lagoons are full of brine shrimps.
0:49:49 > 0:49:53The beta-carotene in these tiny crustaceans
0:49:53 > 0:49:56gives the flamingos their distinctive colour.
0:49:59 > 0:50:03But it will be another two years before he starts to turn pink.
0:50:48 > 0:50:53Here on the Yucatan's coast, as elsewhere on the peninsula,
0:50:53 > 0:50:57all owe their survival to the secret underworld.
0:50:58 > 0:51:01Without its great hidden rivers,
0:51:01 > 0:51:05this would be a hot, dry and hostile place.
0:51:07 > 0:51:10Born of a unique history,
0:51:10 > 0:51:14the underworld hidden beneath this vast forest
0:51:14 > 0:51:17gave rise to the ancient Maya civilisation...
0:51:19 > 0:51:24..and it remains the lifeblood of the Yucatan.
0:51:38 > 0:51:41Capturing the beauty of the Yucatan's underworld
0:51:41 > 0:51:45meant sending a film crew into one of the most alien environments
0:51:45 > 0:51:46on the planet.
0:51:49 > 0:51:52To do this safely required expertise.
0:51:54 > 0:51:58Between them, this team has over 50 years' experience
0:51:58 > 0:51:59diving on the peninsula.
0:52:00 > 0:52:03I want to make sure we get that right, you know?
0:52:03 > 0:52:06Cameraman Mike Madden was one of the pioneers
0:52:06 > 0:52:09of cave exploration here back in the 1980s.
0:52:09 > 0:52:13And Sam Meacham is part of a team currently mapping
0:52:13 > 0:52:16one of the region's two longest caves.
0:52:17 > 0:52:20There's a common misconception that cave divers are these
0:52:20 > 0:52:23adrenaline junkie risk-takers cheating death on every dive.
0:52:24 > 0:52:28Were tarantulas in our risk management form?
0:52:28 > 0:52:32'When, in fact, we're probably some of the most cautious people
0:52:32 > 0:52:33'on the planet.'
0:52:37 > 0:52:40There's a very real need for caution.
0:52:42 > 0:52:44All diving carries risks.
0:52:44 > 0:52:47But cave diving heightens these risks.
0:52:54 > 0:52:58Surfacing isn't an option if you run out of air.
0:53:03 > 0:53:06This particular cave system, known as Taj Mahal,
0:53:06 > 0:53:09was first explored in 1995.
0:53:11 > 0:53:15It has over three miles of flooded passageways,
0:53:15 > 0:53:17some descending almost 30 metres.
0:53:19 > 0:53:22Filming here poses another challenge.
0:53:22 > 0:53:25Most of the cave is pitch black.
0:53:26 > 0:53:29So the first thing the divers need to do
0:53:29 > 0:53:33is set up a highly specialised underwater lighting rig.
0:53:37 > 0:53:40Four 1,200-watt lights.
0:53:42 > 0:53:44100 metres of cable.
0:53:47 > 0:53:51All of it has to be carried into the cave by hand.
0:53:57 > 0:54:01Lighting the caves on this scale is a new challenge for the team
0:54:01 > 0:54:03and draws on all their expertise.
0:54:06 > 0:54:11Since the 1980s, the dedicated work of divers like Sam and Mike
0:54:11 > 0:54:16has helped put nearly 1,000 miles of the Yucatan's underworld on the map.
0:54:17 > 0:54:20But the vast majority remains uncharted.
0:54:21 > 0:54:26It's pretty amazing that 35 years after the beginning
0:54:26 > 0:54:29of cave diving exploration in this area,
0:54:29 > 0:54:32I would say we're still really beginning to understand
0:54:32 > 0:54:34what's going on here.
0:54:35 > 0:54:39And now there's an added urgency to their work.
0:54:43 > 0:54:48Over the last 50 years the Yucatan's population has skyrocketed.
0:54:52 > 0:54:57The city of Cancun is one of the fastest-growing tourist destinations
0:54:57 > 0:55:01in the world, with over five million visitors every year.
0:55:07 > 0:55:10This creates jobs, but the demand for fresh water
0:55:10 > 0:55:13is putting pressure on the peninsula's underground rivers.
0:55:14 > 0:55:18The future of this area is dependent on tourism
0:55:18 > 0:55:23and so how do we sustainably manage these incredible natural resources
0:55:23 > 0:55:27so that Mexico can continue to receive
0:55:27 > 0:55:3110-12% of its gross domestic product through tourism
0:55:31 > 0:55:33on a 100-mile strip of beach?
0:55:35 > 0:55:37That's important to the future of Mexico,
0:55:37 > 0:55:40not only of this region but the country as a whole.
0:55:44 > 0:55:48Maps of the underground river systems are far from complete.
0:55:49 > 0:55:53New construction work may be taking place above caves
0:55:53 > 0:55:55that are yet to be discovered.
0:55:58 > 0:56:05The entire city of Cancun draws its drinking water from 142 cenotes,
0:56:05 > 0:56:08some of which are now at risk of pollution.
0:56:15 > 0:56:18It's a race against time to reveal the importance
0:56:18 > 0:56:20of the Yucatan's hidden underworld.
0:56:27 > 0:56:30Back at Taj Mahal, with the lights in place,
0:56:30 > 0:56:32the crew are ready to begin filming.
0:56:35 > 0:56:38All our focus is going to be this direction today.
0:56:38 > 0:56:42Every image they film must be meticulously planned.
0:56:46 > 0:56:48Get over. More light.
0:56:49 > 0:56:51Less light.
0:56:52 > 0:56:54Once the dive begins,
0:56:54 > 0:56:57all communication is through hand signals.
0:57:10 > 0:57:14At last the team can capture the splendour of the hidden underworld.
0:57:21 > 0:57:24For most of us, this flooded labyrinth
0:57:24 > 0:57:29remains just as inaccessible as it was to the ancient Maya.
0:57:31 > 0:57:36But for the cave divers devoted to exploring this world,
0:57:36 > 0:57:41the hope is that by opening our eyes to its beauty and importance,
0:57:41 > 0:57:44they will help safeguard its future.
0:58:02 > 0:58:07In the next episode, we head to Mexico's remotest region.
0:58:07 > 0:58:09The far north.
0:58:09 > 0:58:13A land of great deserts and rich prairies.
0:58:15 > 0:58:18Home to the toughest of characters...
0:58:19 > 0:58:21..secret worlds...
0:58:22 > 0:58:26..and some of Mexico's rarest wildlife.