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