Natural World (1983–…): Season 23, Episode 11 - Natural World - full transcript

David Attenborough travels along the coast of Southern Africa which has an incredible variety of sharks, over 140 species, from massive great whites and tiger sharks to dozens of tiny sharks with intriguing names like the pyjama shark and leopard cat shark. The answer to why these seas are so rich in predators lies in the unique pattern of warm and cold currents allowing sharks that normally live oceans apart to coexist.

Are you wondering how healthy the food you are eating is? Check it -
Hidden in this jungle

are 3,000 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 are the Yucatan's
only source of freshwater.

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 water holes.

So was the underworld just a myth?

People today can do something
the Maya could only have dreamt about,

breathe underwater.

These modern explorers have made
some remarkable discoveries,

not only about the Maya,

but about the forest,
and its animals, too.

What they've found in the underworld

has changed our understanding
of the Yucatan forever.

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.

SAM MEACHAM: 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 10 years later,
I find myself still here,

so interested and curious
in what I have discovered.

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.

MEACHAM: For me, one of the great
motivating factors in what we do here

is that I'm able to explore
in the 21st century

something I thought
would never have been possible

in my lifetime.

MACCORKINDALE: Just getting to the
cenotes is an adventure in itself.

Like the Maya ruins, they are scattered
over thousands of square kilometres

of trackless forest.

But Sam's not alone.

British-born Steve Bogart
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.

MEACHAM: 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 know where to go
and where not to go,

you can avoid a lot of these problems.

a new, unexplored cenote.

Never mind the jungle tracks.

The real danger for Sam and Steve begins

at the bottom
of these enchanting little pools,

considered sacred by the Maya.

MEACHAM: It's very easy fo 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.

And to walk up to the edge of a cenote

and to look down
into the crystal-clear water

and see the fish swimming below
in the natural daylight,

casting these incredible shafts of light
through the water is very inspiring.

MACCORKINDALE: 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're easy to see.

The jungle does its best
to keep them hidden.

But some animals
you can't help but notice.


Howler monkeys.

Even if you don't see them at first,
you are sure to hear them.

with calls 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 they're 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.



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.

MEACHAM: 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.

And, for me,
that's one of the greatest thrills

about what we do.

-BOGART: Almost.

MACCORKINDALE: Cenotes aren't
just simple pools.

They're caves, flooded caves,
whose roofs have collapsed.

But Sam and Steve have yet to discover

to what extent cenotes are connected
to each other by flooded tunnels.

If there is a network of tunnels
down there, how far does it go?

What they're doing is carefully charting
an as yet uncharted part of the planet.

Somewhere no other human being
has ever gone.

It's one of the riskiest things
an explorer can do.

MEACHAM: 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

MACCORKINDALE: Sometimes there's
hardly enough room to squeeze through.

Getting stuck
or damaging vital equipment now

would be fatal.

MEACHAM: We're diving
in an extremely hostile environment.

It's underwater, it's dark,
it's easy to get disoriented.

And, therefore,
jt'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 are able
to get out of that situation.

MACCORKINDALE: 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 forever.

In a sense,
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.

MEACHAM: 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 a completely
alien and foreign environment

that we don't know a whole lot about.

And many of the caves systems
that we dive in

have seen fewer visitors
than the surface of the Moon.

MACCORKINDALE: It's amazing to think
that a whole civilisation

once sat up 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 to dive

and try to return here over land.

MEACHAM: All right? Looked good, huh?

MACCORKINDALE: 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.

MEACHAM: Hey, Steve,
hang on just a minute.

MACCORKINDALE: The return journey is,
in many ways, more difficult.

GPS puts us right on the target.

MACCORKINDALE: 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, huh?

We should be...

-We should be right here.

Yeah, yeah. That looks promising.
So we're right in the area.

MACCORKINDALE: 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.

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 100 different cave systems.

The promise of future exploration
is high.

There's so much left
that we still have yet to explore.

MACCORKINDALE: 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.

MEACHAM: One of the truisms
of cave diving

js 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 optimal 100% condition

for diving.

MACCORKINDALE: Sam couldn't have
a better dive buddy than Steve.

He's one of the region's
most experienced cave divers

and a master technician.


He knows his equipment inside-out.

Everything good here. You?

-Yeah. Looks good.

Okay, 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 all our life support equipment

and gas supply
is obviously very, very critical to us.

We also use, uh,
a gas management planning rule

known as a rule of thirds.

So we'd use one-third of our gas
swimming into the cave,

one-third swimming back out again,

so that 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.

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.


MACCORKINDALE: 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.

MEACHAM: 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 underwater.

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 will end up
in a dead end

and we turn around and we come back out
and try the other way.

MACCORKINDALE: Using spools of string,

Yucatan's cave divers have measured the
longest underwater cave in the world,

over 133 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're 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.

MEACHAM: It's pretty much guaranteed
that every time

that we go info a cenote,
it's going fo be a different experience.

It's something new,
it's something exciting.

And that's what really draws me in.

One of the many interesting things
of diving here

is to watch all the wildlife
that thrives in the crystal clear water.

MACCORKINDALE: 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 he's 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 sailfin.

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 30 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 caves' 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 has been extinct for 10,000 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 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've 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.

It 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.


Direct descendants
of the ancient Maya still live here.


One of them is Don Fermin Dzip,
a good friend of Sam's.


MACCORKINDALE: The Mayas still practise
slash and burn farming.

Growing crops, then 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 2,000 years.

The Maya may have stopped building
large cities and temples,

but they live on today
as skillful farmers,

thriving, despite the thin soils
and harsh seasons of the Yucatan.


Maya communities are close knit.

And the Mayan language is still spoken.


MACCORKINDALE: "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 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.


MACCORKINDALE: One Maya belief is that
the powerful forces of the underworld

determine their prosperity
and their destiny.


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 safe passage.

And this dive will be difficult.


MACCORKINDALE: 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 sinkholes,

known as pit cenotes.

MEACHAM: This cenote js definitely
a lot deeper

than ones we normally would encounter.

Today, we got to about
45 metres of depth

and still we couldn't see the bottom.

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.

MEACHAM: The hydrogen sulphide layer
js actually pretty intense.

As you're descending down
into the cenofe,

♪ gives the appearance
that you're 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 eggs smell fo 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
20 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
semi-circle 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've led
to the demise of the dinosaurs.

An enormous meteor,

heading for what is now the very tip
of the Yucatan peninsula.

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 jetliner flies today.

MACCORKINDALE: 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 freshwater
is only the top layer.

It floats above an enormous body
of much heavier saltwater.

This is the halocline,
the interface between the two.

It's this contrast
between the gin-clear freshwater

and the hazier saltwater
that can make diving here so surreal.

Divers have discovered
that the freshwater 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
1 go out my door,

there's a new building
that's been built.

MACCORKINDALE: 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
where 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,

and 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.

MACCORKINDALE: 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 lifeblood 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
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 roots of mangroves.

And there are manatees.

These gentle herbivores
come to the underworld's outflow

to drink freshwater 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.

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.

The 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.

MACCORKINDALE: Sam continues
with his passion.

He certainly has his work cut out
for him in the coming years.

MEACHAM: My feelings about exploration
can be very easily summarised

in a poem that I read about
the Yukon Gold Rush.

And in that the author says,

“It's not the gold,
it's finding the gold.”

It's finding the cenote
and diving down into ♪

and seeing what's there
that really is the thrill for me.

Really, for all of us it's a motivation

fo 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.

and his explorer colleagues

have risky yet fascinating days
ahead of them,

unveiling the many secrets
of the Maya underworld.