0:00:21 > 0:00:23Hidden in this jungle
0:00:23 > 0:00:26are three thousand years of human history,
0:00:26 > 0:00:30one of the world's greatest ancient civilisations.
0:00:32 > 0:00:34Here on Mexico's Yucatan Peninsula,
0:00:34 > 0:00:38the Maya built cities, temples and palaces...
0:00:38 > 0:00:42and yet we still don't know how they thrived.
0:00:45 > 0:00:48The forest has grown back
0:00:48 > 0:00:50and nature has taken over again,
0:00:50 > 0:00:53leaving many riddles unsolved,
0:00:53 > 0:00:57like the riddle of the missing river.
0:00:59 > 0:01:05Almost every other ancient civilisation was founded beside a great river.
0:01:06 > 0:01:08But there are none here,
0:01:08 > 0:01:11not even any streams.
0:01:11 > 0:01:16Where is the Nile, the Ganges, or the Euphrates of the Maya?
0:01:21 > 0:01:26What they did have were thousands of these pretty little pools,
0:01:26 > 0:01:28scattered through the jungle.
0:01:28 > 0:01:33Called 'cenotes', they're the Yucatan's only source of fresh water.
0:01:33 > 0:01:37Could they, by themselves, have supported an entire civilisation?
0:01:40 > 0:01:44The Maya believed that cenotes were entrances to another world...
0:01:46 > 0:01:48..an underworld.
0:01:50 > 0:01:55At face value they seem to be little more than beautiful jungle waterholes.
0:01:56 > 0:02:01So was the underworld just a myth?
0:02:01 > 0:02:05People today can do something the Maya could only have dreamt about...
0:02:05 > 0:02:07breathe under water.
0:02:08 > 0:02:14These modern explorers have made some remarkable discoveries, not only about the Maya,
0:02:14 > 0:02:18but about the forest and its animals too.
0:02:18 > 0:02:21What they found in the underworld
0:02:21 > 0:02:25has changed our understanding of the Yucatan for ever.
0:02:42 > 0:02:44The Yucatan,
0:02:44 > 0:02:47a peninsula the size of England,
0:02:47 > 0:02:49separating the Gulf of Mexico from the Caribbean Sea.
0:02:59 > 0:03:02American-born Sam Meacham is a cave diver.
0:03:02 > 0:03:09He's been exploring the waters under the Yucatan for more than a decade...
0:03:09 > 0:03:13but he's still only seen a fraction of what's down there.
0:03:14 > 0:03:19His mission is to explore as many cenotes as he can,
0:03:19 > 0:03:22working with scientists to try to make sense of it all.
0:03:22 > 0:03:26The puzzle of the Yucatan peninsula is extremely complex.
0:03:26 > 0:03:31I arrived here in 1994 with the intention of only being here for six months...
0:03:31 > 0:03:37and ten years later I find myself still here, so interested and curious in what I've discovered.
0:03:39 > 0:03:47But Sam wasn't the first foreign explorer to be drawn to Mexico's jungles by a passion for adventure.
0:03:47 > 0:03:53Back in 1839, John Lloyd Stephens, an American diplomat and travel writer,
0:03:53 > 0:03:58set off into the Yucatan, inspired by rumours of a lost civilisation.
0:04:00 > 0:04:05For a while, he found nothing, even though clues lay all around him.
0:04:09 > 0:04:13Finally, he stumbled upon the ruins of a great city,
0:04:13 > 0:04:16smothered by the jungle.
0:04:23 > 0:04:29The wild tales that Stephens told, made his name as a famous Victorian explorer...
0:04:29 > 0:04:31a hero of his time...
0:04:31 > 0:04:35and, to some, the original Indiana Jones.
0:04:47 > 0:04:54Stephens' fantastic revelations have inspired a whole new generation of explorers.
0:04:54 > 0:04:58For me one of the great motivating factors in what we do here
0:04:58 > 0:05:00is that I am able to explore in the 21st century
0:05:00 > 0:05:04something that I thought would never be possible in my lifetime.
0:05:05 > 0:05:09Just getting to the cenotes is an adventure in itself.
0:05:09 > 0:05:15Like the Maya ruins, they're scattered over thousands of square kilometres of trackless forest.
0:05:15 > 0:05:18But Sam's not alone.
0:05:18 > 0:05:23British-born Steve Bogaerts, shares Sam's passion for exploration.
0:05:23 > 0:05:25They've been cenote-hunting together for years.
0:05:31 > 0:05:37With local help, they mount expeditions deep into the Yucatan's interior.
0:05:37 > 0:05:40It can take days to find a new cenote.
0:05:43 > 0:05:46As we travel through the jungle looking for cenotes,
0:05:46 > 0:05:51of course there's always the usual assembly of spiny trees and cactuses.
0:05:51 > 0:05:54We have crocodiles, we have snakes, scorpions, tarantulas...
0:05:54 > 0:05:56you name it, it's all there.
0:05:56 > 0:06:00But really if you know what to look for and where to go and where not to go,
0:06:00 > 0:06:02you can avoid a lot of these problems.
0:06:16 > 0:06:19Finally, a new, unexplored cenote.
0:06:20 > 0:06:22Never mind the jungle treks,
0:06:22 > 0:06:27the real danger for Sam and Steve begins at the bottom of these enchanting little pools,
0:06:27 > 0:06:29considered sacred by the Maya.
0:06:29 > 0:06:35It's very easy to see how the ancient Maya would have perceived the cenotes as very sacred spaces.
0:06:35 > 0:06:41They're absolutely beautiful jewels out in the middle of this jungle.
0:06:41 > 0:06:46To walk up to the edge of a cenote and to look down into the crystal clear water
0:06:46 > 0:06:47and see the fish swimming below,
0:06:47 > 0:06:53and the natural daylight casting these incredible shafts of light through the water is very inspiring.
0:06:55 > 0:07:01As the sole sources of water in this jungle, these pools are also magnets for wildlife...
0:07:01 > 0:07:05and to cenote specialists like grebes, their whole world.
0:07:12 > 0:07:17With thick forest on all sides, they seem as isolated as islands in an ocean.
0:07:31 > 0:07:36Peccaries, deer and other forest animals use cenotes as watering holes.
0:07:36 > 0:07:39But that doesn't mean they are easy to see.
0:07:39 > 0:07:41The jungle does its best to keep them hidden.
0:07:50 > 0:07:53But some animals you can't help but notice...
0:07:53 > 0:07:55ANIMAL ROARS
0:08:00 > 0:08:02..howler monkeys.
0:08:03 > 0:08:08Even if you don't see them at first, you are sure to hear them.
0:08:08 > 0:08:13With call that carry five kilometres, they're the loudest land animals in the world.
0:08:18 > 0:08:20Howlers are sloppy eaters.
0:08:20 > 0:08:26Coatis following below can fill their stomachs solely out of what they've dropped.
0:08:30 > 0:08:31Spider monkeys.
0:08:31 > 0:08:37They're infinitely quieter than howlers but are much more agile.
0:08:37 > 0:08:39With their hooking hands and long arms,
0:08:39 > 0:08:43these monkeys can live their entire lives in the dense forest canopy.
0:08:48 > 0:08:52For nine months of the year, there is no rainfall here
0:08:52 > 0:08:55and much of the forest struggles to survive.
0:08:57 > 0:09:01But some trees seem immune to the drought.
0:09:01 > 0:09:04What's their secret?
0:09:04 > 0:09:06Such are the riddles of the Yucatan.
0:09:08 > 0:09:12The answers lie underground.
0:09:12 > 0:09:16But Sam and Steve won't get to go there until tomorrow.
0:09:28 > 0:09:30In the dark,
0:09:30 > 0:09:35the jungle seems even denser and the sounds even stranger.
0:09:40 > 0:09:43This is when cenotes really come alive.
0:09:46 > 0:09:50Tapirs love water... for bathing as much as drinking...
0:09:50 > 0:09:54but visiting a cenote means coming into the open,
0:09:54 > 0:09:58which for good reason, they only do after dark.
0:10:08 > 0:10:14Like watering holes anywhere, cenotes are where predators, in this case, jaguars, come to hunt.
0:10:32 > 0:10:33ANIMAL CRY OF ALARM
0:10:47 > 0:10:51But to the Maya, cenotes were more than just jungle watering holes...
0:10:52 > 0:10:54..they were central to their world.
0:10:56 > 0:11:00Cities and temples were often built right next to them.
0:11:03 > 0:11:07These sacred wells were gateways to the underworld,
0:11:07 > 0:11:10a terrifying place of spirits
0:11:10 > 0:11:13and of fearsome gods who demanded respect.
0:11:18 > 0:11:24At the bottom of many cenotes lie offerings made to the underworld.
0:11:30 > 0:11:34For archaeologists cenotes are time capsules
0:11:34 > 0:11:38that provide clues to how the ancient Maya lived...and died.
0:11:43 > 0:11:49Sometimes even the people themselves were sacrificed to the gods they feared so much.
0:11:49 > 0:11:51Every pot and skeleton
0:11:51 > 0:11:53has its own story to tell.
0:12:06 > 0:12:09The discoveries of underwater explorers
0:12:09 > 0:12:14are helping archaeologists rewrite the Yucatan's ancient history.
0:12:31 > 0:12:34Yucatan's explorers aren't just interested
0:12:34 > 0:12:39in the clues to Maya history that they might find at the bottom of these pools.
0:12:39 > 0:12:43They want to know what might lie beyond them.
0:12:44 > 0:12:46Is there, indeed,
0:12:46 > 0:12:48an underworld?
0:12:51 > 0:12:55Could this cenote be a gateway to a whole new world?
0:12:55 > 0:12:56If it is,
0:12:56 > 0:12:59where does that world lead?
0:12:59 > 0:13:01Every new cenote
0:13:01 > 0:13:04presents a new opportunity.
0:13:04 > 0:13:09Cenotes really present us with the truest form of exploration found today.
0:13:09 > 0:13:11When we come up to the side of a cenote
0:13:11 > 0:13:16we literally have no idea what we're going to find at the bottom of it,
0:13:16 > 0:13:18until we actually get in and investigate.
0:13:18 > 0:13:22For me, that's one of the greatest thrills about what we do.
0:13:26 > 0:13:28Cenotes aren't just simple pools.
0:13:28 > 0:13:32They're caves, flooded caves whose roofs have collapsed.
0:13:32 > 0:13:38But Sam and Steve have yet to discover is to what extent cenotes are connected to each other
0:13:38 > 0:13:39by flooded tunnels.
0:13:39 > 0:13:44If there is a network of tunnels down here, how far does it go?
0:13:50 > 0:13:55What they're doing is carefully charting an as-yet uncharted part of the planet,
0:13:55 > 0:13:58somewhere no other human has ever gone.
0:14:00 > 0:14:04It's one of the riskiest things an explorer can do.
0:14:23 > 0:14:26This type of diving isn't for everybody
0:14:26 > 0:14:31and definitely you have to want to do it in order to be involved in it.
0:14:31 > 0:14:35The first cave dive that I ever did, actually,
0:14:35 > 0:14:37I was pretty nervous.
0:14:37 > 0:14:42Talk to an astronaut that sat on top of a rocket full of fuel and blasted off to the moon,
0:14:42 > 0:14:47sure I bet they were a little bit nervous, but look what we've gained through space exploration.
0:14:47 > 0:14:51All those people were willing to take a risk to achieve an incredible goal.
0:14:55 > 0:15:00Sometimes there's hardly enough room to squeeze through.
0:15:00 > 0:15:05Getting stuck or damaging vital equipment now would be fatal.
0:15:05 > 0:15:08We're diving in an extremely hostile environment.
0:15:08 > 0:15:12It's under water, it's dark, it's easy to get disorientated,
0:15:12 > 0:15:16and, therefore, it's easy to have panic attacks.
0:15:16 > 0:15:20There's two ways out of a panic situation,
0:15:20 > 0:15:21luck and death,
0:15:21 > 0:15:25and therefore panic is not an option for us.
0:15:25 > 0:15:28You really have to take three deep breaths, calm yourself
0:15:28 > 0:15:33and assure yourself that you're able to get out of that situation.
0:15:54 > 0:15:58Exploration is rarely without risks.
0:15:59 > 0:16:05But one of the biggest rewards is seeing something that's never been seen before.
0:16:05 > 0:16:10What they've discovered down here, is just staggering.
0:16:30 > 0:16:32The Maya did have an underworld,
0:16:32 > 0:16:38and it's as strange and as beautiful a place as any myth might describe.
0:16:46 > 0:16:50They've revealed a vast system of flooded caves,
0:16:50 > 0:16:53underpinning much of the peninsula.
0:16:53 > 0:16:57It's changed our view of the Yucatan for ever.
0:17:06 > 0:17:09In a way, this is like exploring outer space,
0:17:09 > 0:17:12the weightlessness,
0:17:12 > 0:17:14the utter strangeness,
0:17:14 > 0:17:17the thrill of the unknown.
0:17:19 > 0:17:22Cave divers call this "inner space".
0:17:24 > 0:17:28Sam has got close to a long-held ambition.
0:17:28 > 0:17:32One of my childhood dreams was to become an astronaut.
0:17:32 > 0:17:33I'm not an astronaut now,
0:17:33 > 0:17:39but I feel that I'm as close as I can come to outer space exploration in the work that we do here.
0:17:39 > 0:17:42We're completely dependent on life-support equipment,
0:17:42 > 0:17:47we travel into an alien and foreign environment that we don't know a whole lot about
0:17:47 > 0:17:53and many of the cave systems that we dive in have seen fewer visitors than the surface of the moon.
0:17:53 > 0:17:58It's amazing to think that a whole civilisation once sat on top of all this,
0:17:58 > 0:18:01trying to imagine what was down here.
0:18:04 > 0:18:09The reality of this place can be as surreal as anything the Maya may have dreamt of.
0:18:12 > 0:18:14Sometimes what seems to be air...
0:18:14 > 0:18:16isn't.
0:18:20 > 0:18:23It's just a different kind of water.
0:18:31 > 0:18:35Some caves contain layers of water that just don't mix.
0:18:35 > 0:18:38There's so much about this system that we don't yet understand.
0:18:38 > 0:18:43Sam and Steve's aim is to find out how it all connects.
0:18:43 > 0:18:44They're making maps.
0:18:53 > 0:18:56Light ahead reveals a new cenote.
0:19:00 > 0:19:04They'll record its position, then swim back to where they started the dive
0:19:04 > 0:19:06and try to return here overland.
0:19:12 > 0:19:16The more they explore, the more connections they find.
0:19:16 > 0:19:19But they've got a long way to go.
0:19:19 > 0:19:22There are still thousands of cenotes left to investigate.
0:19:30 > 0:19:33The return journey is in many ways more difficult.
0:19:36 > 0:19:39Underground, they went where the tunnels led them.
0:19:39 > 0:19:46Up here, they're looking for one tiny pool among thousands, hidden somewhere in a dense jungle.
0:19:46 > 0:19:51For this, they'll need satellite positioning and aerial photographs.
0:19:51 > 0:19:54- Yeah, GPS puts us right... - Right on it, eh?
0:19:54 > 0:19:56We should be...
0:19:59 > 0:20:01- We should be right here.- Uh-huh.
0:20:01 > 0:20:05Yeah, looks promising. So we're right in the area.
0:20:05 > 0:20:07State of the art technology gets them close,
0:20:07 > 0:20:12but on the final stretch, they get a helping hand from birds.
0:20:14 > 0:20:17Turquoise-browed motmots.
0:20:17 > 0:20:19These are true cenote birds.
0:20:19 > 0:20:25They feed on the abundant insects near the water and often nest inside the caves.
0:20:26 > 0:20:28BIRD CALLS
0:20:34 > 0:20:39Their distinctive call almost always means there's a cenote nearby.
0:20:43 > 0:20:47It was the ancient Maya who first used them as guides to water.
0:20:47 > 0:20:49This works just as well today.
0:20:57 > 0:21:02Now they've located the new cenote, Sam and Steve need to find out
0:21:02 > 0:21:06if it has further connections with other parts of the system.
0:21:07 > 0:21:10In our corner of the Yucatan peninsula,
0:21:10 > 0:21:14the collaborative efforts of cave-diving explorers
0:21:14 > 0:21:21have mapped and explored over 550 kilometres of underground, underwater passageway
0:21:21 > 0:21:24in over a hundred different cave systems.
0:21:24 > 0:21:28The promise of future exploration is high,
0:21:28 > 0:21:32there's so much left that we still have yet to explore.
0:21:39 > 0:21:45This may seem like nothing more than an elaborate game of join the dots,
0:21:45 > 0:21:48but each time Sam and Steve go back underground,
0:21:48 > 0:21:52they never lose sight of the potential dangers of their work.
0:21:52 > 0:21:57One of the truisms of cave diving is that complacency breeds death
0:21:57 > 0:22:02and every single dive we approach as if it's the first dive we've done.
0:22:02 > 0:22:06And we have a ritual that we go through of matching our gear,
0:22:06 > 0:22:12checking for leaks and making sure that everything is in optimum, 100% condition for diving.
0:22:14 > 0:22:16Sam couldn't have a better dive buddy than Steve.
0:22:16 > 0:22:22He is one of the region's most experienced cave divers, and a master technician.
0:22:22 > 0:22:24He knows his equipment inside out.
0:22:24 > 0:22:28- Everything good here? - Yes. Looks good.- OK.
0:22:29 > 0:22:35OK, one of the first things you'll notice is that we're actually taking two tanks with us, rather than one.
0:22:35 > 0:22:39That's because we're diving in an alien, potentially hostile environment
0:22:39 > 0:22:42and we need redundancy in our life-support equipment
0:22:42 > 0:22:45and gas supply is obviously very, very critical to us.
0:22:45 > 0:22:49We also use a gas management planning rule, known as The Rule Of Thirds,
0:22:49 > 0:22:54so we use one third of our gas swimming into the cave, one third swimming out again.
0:22:54 > 0:22:58When we surface we have one third in reserve and that's an emergency reserve,
0:22:58 > 0:23:02should it take us longer to exit than we anticipated, or if we needed to share air with a buddy.
0:23:10 > 0:23:18A thin piece of white string, carefully laid, quite literally becomes their lifeline.
0:23:18 > 0:23:21It may be the only way that they can find their way back,
0:23:21 > 0:23:23out of the labyrinth.
0:23:37 > 0:23:42They mark it with arrows that always point back towards the entrance, and safety.
0:23:45 > 0:23:47It's also a measuring tape.
0:23:47 > 0:23:51Regularly spaced knots tell Sam and Steve how far they've gone.
0:23:57 > 0:24:01As we explore the cave systems, we try to be as smart as we can,
0:24:01 > 0:24:07and generally we're trying to go in a particular direction and we have compasses that work under water
0:24:07 > 0:24:12and using those compasses we're able to determine which route to take.
0:24:16 > 0:24:21It's quite common to come up to a split in a passageway.
0:24:21 > 0:24:25We have to determine which is the best route to take.
0:24:25 > 0:24:30In some cases, that'll end up in a dead end and we turn around and come back out and try the other way.
0:24:39 > 0:24:40Using spools of string,
0:24:40 > 0:24:45Yucatan's cave divers have measured the longest underwater cave in the world,
0:24:45 > 0:24:48over one hundred and thirty-three kilometres long.
0:24:51 > 0:24:56Exploration wouldn't be exploration if everything always went to plan.
0:24:56 > 0:25:02This time the divers have come to a passage too tight to squeeze through
0:25:02 > 0:25:04and they are forced to stop.
0:25:06 > 0:25:10They follow their safety line back and live to dive another day.
0:25:10 > 0:25:16But explorers wouldn't be explorers if they let such setbacks discourage them.
0:25:16 > 0:25:19There's always the thrill of the next dive.
0:25:19 > 0:25:24It's pretty much guaranteed that every time that we go into a cenote,
0:25:24 > 0:25:27it's going to be a different experience.
0:25:27 > 0:25:32It's something new, something exciting and that's what really draws me in.
0:25:32 > 0:25:35One of the many interesting things here is to watch all the wildlife
0:25:35 > 0:25:38that thrives in the crystal clear water.
0:25:38 > 0:25:41That includes sailfin mollies,
0:25:41 > 0:25:46small fish that stick to the bright sunlit zones in the open water pools of cenotes.
0:25:48 > 0:25:50For a male, it's a hectic life.
0:25:50 > 0:25:53He has a three-dimensional territory to patrol
0:25:53 > 0:25:59and is constantly chasing other males out while trying to keep his harem of females in.
0:26:04 > 0:26:09In both cases, success depends on how effectively he displays his sail fin.
0:26:11 > 0:26:14It's a big job for a little fish.
0:26:30 > 0:26:36Some fish, like these tetras, have proved to be real opportunists.
0:26:36 > 0:26:41They've learnt to follow divers' torches into the dark to feed right inside the caves.
0:26:45 > 0:26:50Our divers take care not to bring any uninvited guests with them
0:26:50 > 0:26:54because the underworld has its own unique creatures,
0:26:54 > 0:27:01an entire food chain of over thirty species that live out their lives in the pitch dark.
0:27:08 > 0:27:11Most cave animals are white
0:27:11 > 0:27:15because in a world without light, colour is pointless.
0:27:15 > 0:27:20Even eyes are useless, and many creatures just don't have them.
0:27:22 > 0:27:27Down here, touch and smell are all that matter.
0:27:29 > 0:27:35Among the strangest and most ancient of cave beasts is the remipede,
0:27:35 > 0:27:41a sort of primitive centipede that's rarely seen, found only in waters exceptionally low in oxygen.
0:27:41 > 0:27:44Relics of one of the earliest chapters of life on earth,
0:27:44 > 0:27:49they're among the cave's top predators, combing the water for shrimps and isopods.
0:27:52 > 0:27:56If the remipede doesn't seem to know which way is up,
0:27:56 > 0:28:02that's because, in the water and in the dark, "up" and "down" aren't so relevant.
0:28:15 > 0:28:19In the underworld, even the fish are surreal...
0:28:19 > 0:28:23ghostly white with blanks where eyes should be.
0:28:27 > 0:28:29There are other signs of life down here.
0:28:33 > 0:28:38This is the perfectly preserved tooth of a Gomphotherium,
0:28:38 > 0:28:42a relative of the elephant that's been extinct for ten thousand years.
0:28:48 > 0:28:52Ancient animal remains - and these stalactites and stalagmites -
0:28:52 > 0:28:55only ever formed in air,
0:28:55 > 0:29:00are hard evidence that these caves used to be dry.
0:29:00 > 0:29:03And Yucatan's history goes deeper still.
0:29:03 > 0:29:10The walls of these caves are made of soft limestone, telling us that this was once a huge coral reef.
0:29:25 > 0:29:32Some caves near the surface have air pockets and cracks in their ceilings that allow bats to come and go.
0:29:32 > 0:29:34Cave swifts too.
0:29:34 > 0:29:38It's the perfect, sheltered place to roost and nest.
0:29:38 > 0:29:42No wonder the Maya thought bats were from the underworld.
0:29:42 > 0:29:46They would have seen them flying straight out of the ground as night fell.
0:29:49 > 0:29:52By exploring underground,
0:29:52 > 0:29:55Yucatan's divers are peeling back the many layers of the peninsula
0:29:55 > 0:29:58and are slowly revealing the incredible relationship
0:29:58 > 0:30:03between its flooded caves and everything they affect at the surface.
0:30:06 > 0:30:09There are many ways in which these two worlds connect...
0:30:11 > 0:30:13..tree roots.
0:30:15 > 0:30:18This, is the jungle's secret...
0:30:18 > 0:30:22how, with hardly any surface water, it can still grow so dense.
0:30:24 > 0:30:31Some trees and vines push their roots through gaps in the limestone to the permanent water supply below.
0:30:33 > 0:30:39It doesn't matter how dry it gets on the surface, they rely on the underworld.
0:30:39 > 0:30:42These deep-rooted trees provide animals
0:30:42 > 0:30:47with a year-round supply of leaves, flowers and fruit.
0:30:54 > 0:31:01This vital connection between the forest and the ground beneath it must have intrigued the Maya
0:31:01 > 0:31:05and could only have reinforced their belief in the power of the underworld.
0:31:07 > 0:31:10They too relied on its gift of water.
0:31:10 > 0:31:16A few cenotes could help a whole city survive even the harshest of dry seasons.
0:31:24 > 0:31:30But Sam doesn't just look to archaeology for his understanding of the Maya.
0:31:30 > 0:31:32He can talk to them.
0:31:32 > 0:31:33Ho!
0:31:33 > 0:31:37Direct descendants of the ancient Maya still live here.
0:31:39 > 0:31:44One of them is Don Fermin Dzip, a good friend of Sam's.
0:31:45 > 0:31:47HE SPEAKS SPANISH
0:31:51 > 0:31:54The Maya still practise slash and burn farming,
0:31:54 > 0:31:59growing crops and letting the forest grow back to replenish the soil.
0:31:59 > 0:32:03In fact, the ancient Maya did this on a grand scale.
0:32:03 > 0:32:07Incredibly, most of the jungle here, previously thought to be pristine,
0:32:07 > 0:32:12has actually been cut down and re-grown many times over the last two thousand years.
0:32:16 > 0:32:22The Maya may have stopped building large cities and temples, but they live on today as skilful farmers,
0:32:22 > 0:32:26thriving despite the thin soils and harsh seasons of the Yucatan.
0:32:28 > 0:32:30Buenas tardes!
0:32:30 > 0:32:32Buenas tardes!
0:32:42 > 0:32:47Maya communities are close knit, and the Mayan language is still spoken.
0:33:03 > 0:33:08"Cenote" is derived from the Maya word for "well".
0:33:08 > 0:33:10Almost every village is built around one.
0:33:10 > 0:33:14Other cenotes mark boundaries between the communities.
0:33:22 > 0:33:27Cenotes were, and are, quite literally central to their world.
0:33:35 > 0:33:40As well as a distinct language, the Maya have a distinct set of beliefs.
0:33:40 > 0:33:46Their stories and fables, passed down the generations, describe everything around them...
0:33:46 > 0:33:48the cenotes,
0:33:48 > 0:33:50the jungle,
0:33:50 > 0:33:51the animals.
0:33:55 > 0:34:01One Maya belief is that the powerful forces of the underworld determine their prosperity...
0:34:01 > 0:34:03and their destiny.
0:34:04 > 0:34:06CHANTING
0:34:09 > 0:34:13Don Fermin still practises the Maya religion.
0:34:13 > 0:34:20He prays to the gods of his ancestors and regards cenotes as windows into their world.
0:34:25 > 0:34:28In advance of Sam's more difficult dives,
0:34:28 > 0:34:35Don Fermin sometimes makes offerings to the underworld, asking for a safe passage.
0:34:35 > 0:34:37And this dive will be difficult.
0:34:41 > 0:34:46But it will reveal yet another twist in the Yucatan's many layered history,
0:34:46 > 0:34:50a cosmic event that affected not only the world of the ancient Maya,
0:34:50 > 0:34:54but possibly the rest of the world as well.
0:34:59 > 0:35:02Some cenotes near the north-western tip of the Yucatan
0:35:02 > 0:35:07aren't at all like the ones Sam and Steve are used to exploring.
0:35:07 > 0:35:12These are much deeper, sheer, vertical sink holes...
0:35:12 > 0:35:14known as pit cenotes.
0:35:22 > 0:35:26This cenote is, is definitely a lot deeper than, than the ones we would normally encounter.
0:35:26 > 0:35:31Today we got to about 45m in depth and still couldn't see the bottom.
0:35:38 > 0:35:41This appears to be the bottom,
0:35:41 > 0:35:43but it isn't.
0:35:43 > 0:35:47It's a cloud of hydrogen sulphide, made from rotting vegetation.
0:35:50 > 0:35:53It's toxic and corrosive...
0:35:53 > 0:35:55not somewhere you'd want to hang around.
0:36:04 > 0:36:07The hydrogen sulphide layer is actually pretty intense.
0:36:07 > 0:36:12As you are descending down into the cenote, it gives the appearance that you are coming up on the floor.
0:36:12 > 0:36:16And all of a sudden you realise it's not the floor, it's a cloud.
0:36:16 > 0:36:20It's made up of sulphur primarily, so it's got a rotten-egg smell to it.
0:36:20 > 0:36:25In extreme cases, where it's very strong, you can feel it burning any exposed skin that you have.
0:36:46 > 0:36:49Why are these cenotes so different?
0:36:49 > 0:36:54They're evidence of a critical turning point in the Yucatan's distant history,
0:36:54 > 0:36:57something that was only noticed twenty years ago,
0:36:57 > 0:37:01when satellites gave us a new perspective on life on Earth.
0:37:06 > 0:37:12If you look at normal cenotes from space, their pattern is scattered and random.
0:37:14 > 0:37:20But the pit cenotes form a distinct semicircle 165 kilometres across.
0:37:23 > 0:37:28Seismic studies have shown that the circle is completed under the sea.
0:37:28 > 0:37:32So what does this huge circle represent?
0:37:32 > 0:37:36The answer lies at least 65 million years ago,
0:37:36 > 0:37:41when the Yucatan was a shallow tropical sea.
0:37:41 > 0:37:45The disastrous event that caused the circle was so massive
0:37:45 > 0:37:49that some think it could have led to the demise of the dinosaurs.
0:37:51 > 0:37:53An enormous meteor
0:37:53 > 0:37:58heading for what is now the very tip of the Yucatan Peninsula.
0:38:08 > 0:38:09EXPLOSION
0:38:14 > 0:38:20Imagine at the moment that this meteor slammed into our planet,
0:38:20 > 0:38:24it was so huge that if one edge of it was touching our planet,
0:38:24 > 0:38:30the outer edge of it would be at the same altitude as a commercial jet liner flies today.
0:38:30 > 0:38:34The immense impact crater was gradually buried under limestone,
0:38:34 > 0:38:38built up by coral reefs over millions of years.
0:38:38 > 0:38:42But the crater's shape was echoed in the way this limestone then eroded
0:38:42 > 0:38:46to form the distinctive semi-circle of pit cenotes.
0:38:47 > 0:38:53When the Maya arrived, they built great cities and temples around these "sacred wells",
0:38:53 > 0:38:58unwittingly outlining the footprint of this global catastrophe.
0:39:01 > 0:39:06Once again, the Yucatan's history can be read by looking deep into its landscape.
0:39:10 > 0:39:13But it has one more secret to reveal,
0:39:13 > 0:39:16one last riddle to be solved.
0:39:28 > 0:39:30When it does rain here,
0:39:30 > 0:39:32it rains hard.
0:39:36 > 0:39:38But this huge amount of water
0:39:38 > 0:39:41doesn't settle on the ground.
0:39:41 > 0:39:43It vanishes.
0:39:44 > 0:39:46It seeps through the limestone
0:39:46 > 0:39:48into the underworld.
0:39:50 > 0:39:55But this fresh water is only the top layer.
0:39:55 > 0:40:00It floats above an enormous body of much heavier salt water.
0:40:00 > 0:40:02This is the halocline,
0:40:02 > 0:40:04the interface between the two.
0:40:06 > 0:40:09It's this contrast between the gin-clear fresh water
0:40:09 > 0:40:14and the hazier salt water that can make diving here so surreal.
0:40:20 > 0:40:24Divers have discovered that the fresh water here
0:40:24 > 0:40:27does more than just float.
0:40:30 > 0:40:34It flows... in huge underground rivers,
0:40:34 > 0:40:38probably the largest underground river system in the world.
0:40:45 > 0:40:48Nearly two centuries ago,
0:40:48 > 0:40:52John Lloyd Stephens rediscovered the Maya civilisation.
0:40:53 > 0:40:57People have long wondered how they thrived without a great river.
0:40:59 > 0:41:00Now...
0:41:00 > 0:41:03we appear to have found their Nile.
0:41:07 > 0:41:11These great rivers must flow out to sea, but where?
0:41:11 > 0:41:13Sam needs to find out.
0:41:16 > 0:41:20He comes across the skeleton of a manatee, a sea mammal.
0:41:20 > 0:41:22He must be getting close.
0:41:32 > 0:41:36Metre by metre, cenote to cenote,
0:41:36 > 0:41:40cave divers are mapping the rivers from source to sea.
0:41:43 > 0:41:45But while doing so,
0:41:45 > 0:41:48they've made an alarming discovery.
0:41:54 > 0:41:56The modern world is taking over.
0:42:00 > 0:42:05I'm amazed at the changes that have taken place in such a short time in this area.
0:42:05 > 0:42:10It seems that every time I go out of my door, there's a new building that's been built.
0:42:10 > 0:42:14The coastal strip of Cancun and the Riviera Maya
0:42:14 > 0:42:17is one of the fastest-growing tourist areas in the world.
0:42:17 > 0:42:23There's one specific occasion when we were actually diving beneath a major construction project
0:42:23 > 0:42:29and, as we were diving along, the entire cave was literally shaking as we were diving through it.
0:42:29 > 0:42:33And it wasn't until the next day that we came back, that we realised
0:42:33 > 0:42:37that they had been perforating through the ceiling of the cave.
0:42:37 > 0:42:40Along one of the lines that Steve had laid the previous day,
0:42:40 > 0:42:45there was actually a cement piling going right down through the cave system.
0:42:45 > 0:42:52New construction could inadvertently block or pollute the great underground rivers of the Yucatan,
0:42:52 > 0:42:56with far-reaching effects still too complex for us to understand.
0:42:56 > 0:43:01The Maya underworld faces a new chapter in its long and varied history.
0:43:06 > 0:43:10The decline of the ancient Maya could teach us a thing or two.
0:43:12 > 0:43:15Some say they developed too far, too fast.
0:43:15 > 0:43:21Others that a succession of droughts left them without water.
0:43:25 > 0:43:30Everyone here still relies on the underworld.
0:43:30 > 0:43:34It is, and always was, the life-blood of the peninsula.
0:43:34 > 0:43:40Without it, the Yucatan would be a hot, dry and hostile place.
0:43:46 > 0:43:53By mapping the course of every river to the sea, Sam and other divers are hoping to draw attention to them,
0:43:53 > 0:43:55so further damage can be avoided.
0:44:02 > 0:44:06Their work has not only helped us to understand the Yucatan's past,
0:44:06 > 0:44:09but it can help to safeguard its future.
0:44:15 > 0:44:17Sam's journey down this river
0:44:17 > 0:44:19is nearly over.
0:44:19 > 0:44:22There's more light, and more air
0:44:22 > 0:44:26and the roots are the roots of mangroves.
0:44:27 > 0:44:29And there are manatees.
0:44:33 > 0:44:40These gentle herbivores come to the underworld's outflow to drink fresh water and to cool off.
0:44:40 > 0:44:43What they mean to Sam is that he's made it.
0:44:56 > 0:44:58One last tunnel...
0:45:02 > 0:45:07..and a journey that began in a jungle pool ends up off a Caribbean beach.
0:45:09 > 0:45:13Tomorrow, he'll be back in the forest, looking for a new cenote
0:45:13 > 0:45:15and the next river.
0:45:15 > 0:45:19And when all the cenotes are explored and all the maps are finished,
0:45:19 > 0:45:21maybe the Yucatan will be better understood.
0:45:29 > 0:45:34In a more mystical way, the ancient Maya understood it.
0:45:34 > 0:45:37They knew they were at the mercy of the underworld.
0:45:41 > 0:45:47At the ruins, archaeologists are revealing ever more about this great civilisation,
0:45:47 > 0:45:49how they lived and what they believed.
0:45:53 > 0:45:57But now, a whole new frontier has opened up underground.
0:46:05 > 0:46:11Sam and Steve are not the first explorers to have been enchanted by the riddles of the Yucatan,
0:46:11 > 0:46:16but they have, quite literally, taken exploration to a whole new level.
0:46:19 > 0:46:24To this day it's only thought that we've charted a fraction of the Maya underworld
0:46:24 > 0:46:28and many of these areas still remain untouched and uncharted.
0:46:33 > 0:46:35Sam continues with his passion.
0:46:35 > 0:46:39He certainly has his work cut out for him in the coming years.
0:46:42 > 0:46:49My feelings about exploration can be very easily summarised in a poem that I read about the Yukon gold rush.
0:46:49 > 0:46:54In that, the author says, "It's not the gold, it's finding the gold."
0:46:55 > 0:47:01It's finding the cenote and diving down into it and seeing what's there that really is the thrill for me.
0:47:04 > 0:47:07Really, for all of us, it's a motivation to think
0:47:07 > 0:47:12that you can live in the 21st century and still be able to explore.
0:47:12 > 0:47:15We're only just scratching the surface of what exists here.
0:47:15 > 0:47:20I have absolutely no doubt that this place will continue to provide
0:47:20 > 0:47:23incredible scientific discoveries for years to come.
0:47:26 > 0:47:33Sam and his explorer colleagues have risky, yet fascinating days ahead of them,
0:47:33 > 0:47:37unveiling the many Secrets Of The Maya Underworld.
0:48:08 > 0:48:10Subtitles by BBC Broadcast 2005
0:48:10 > 0:48:13E-mail us at subtitling@bbc.co.uk