Original subtitles

DAVID SCHWIMMER: Our planet still hides many mysteries.

In 2009, marine biologists discover

something extraordinary--

whale sharks, more than 400 of them

all in the same place just off the coast

of Mexico, the largest gathering of the world's

largest fish ever seen.

But why are they here?

The all-inclusive resorts?

To find out, we need to travel back in time

to a lost civilization and its sacred underworld, home

to monsters and alien life forms,

to the clashing of continents, and to the day when

a cosmic catastrophe completely rewrites the story of life

on Earth.

Hi.

Don't mind the book.

I'm Detective David Schwimmer, and it's

a crime you haven't watched this episode of "Mysterious Planet--

Giants of the Caribbean."

Can someone get the book?

The largest fish on Earth cruises

through the clear waters of the Caribbean.

A whale shark, 25-feet long, as big as a bus--

or twice the size of a New York apartment.

Unlike other sharks known for their killer instinct

and lethal jaws, this one's taken a very

different evolutionary path.

It has teeth, around 3,000 of them, but they are tiny,

and it doesn't use them to eat or to open

difficult bags of chips.

Its mouth, however, is huge--

over 5-feet wide.

The world's biggest shark is a filter feeder.

Like a giant underwater vacuum cleaner, it sucks in water

and filters out microscopic plankton using spongy pads

hidden behind its gill slits.

It can filter two Olympic-sized swimming

pools of water every day.

Just like me, whale sharks are loners, constantly on the move,

wandering the tropical oceans of the world.

Now something is calling these solitary creatures

together to the largest gathering

of their kind on Earth.

But the whale shark's story doesn't start on Earth.

It starts one day 66 million years ago in outer space.

Space sharks!

It's one of the worst days in our planet's history

that wasn't my 11th birthday.

A piece of rock the size of Manhattan

has a date with destiny, a collision

course with planet earth.

The mega disaster that follows changes life on Earth forever.

Millions of years later, it helps create the world's

largest gathering of whale sharks

in the very same place where it lands,

Mexico's Yucatan Peninsula.

Just off the coast of the Yucatan

is an underwater wonderland.

The sun, and clear, shallow water,

create the perfect conditions for some of the most

extensive coral reefs on Earth.

They home to over 500 species of fish

and a bounty of food for hungry grazers.

Not a coincidence.

But further out beyond the reef, the Caribbean Sea

is a blue desert.

The water is so clear because it contains

almost no sediment or plankton.

And yet, hundreds of whale sharks are heading this way--

and that guy.

Why are they traveling thousands of miles

to reach a place where there appears to be no food for them?

Also known as going home for the holidays.

The search for answers begins on land or, more precisely,

underneath it.

Beyond the coast, the Yucatan Peninsula

is a wilderness of lush rainforest,

flat and featureless like a green carpet with no rivers

or streams anywhere in sight.

Yet a mysterious civilization called the Maya thrived

here for a thousand years.

How did they survive with no obvious source of fresh water?

The answer lies in these strange, round pools.

There's another.

Known as cenotes, they are the key to all life

on the Yucatan, the only source of drinking

water for animals and humans.

The Maya built their cities and temples

around these natural reservoirs, and they held

great spiritual significance.

Cenotes were passages to the underworld, the crossroads

between the living and the dead, places of ritual

and human sacrifice.

OK, I didn't expect that part.

But they really are portals to another realm

for those brave enough to enter, like Derek.

Underneath the forests of the Yucatan

is one of the wonders of the natural world,

the largest underwater cave system on Earth.

Divers recently discovered that one

of the largest cenote systems is over 200-miles long.

But to fully understand the whale shark mystery,

we'll have to go even deeper.

Off Mexico's Yucatan Peninsula, an incredible event

is about to happen, the world's largest

gathering of whale sharks.

It's like Woodstock for whale sharks.

Vital clues to this biological mystery

are hidden nearby in a maze of water-filled caves.

It's an alien realm down here with unearthly landscapes,

giant caves decorated with ornate stone sculptures

like otherworldly cathedrals.

How was this vast underwater labyrinth created?

As divers venture deeper into places that have never

been seen before, they swim back in time, which

should be an Olympic event.

Fossilized coral hidden in the stone

walls, proof that these rocks were formed under the ocean.

But the spectacular stalactites only form in air,

so what gives?

The only explanation for these conflicting clues?

The Yucatan and its unique cenote network

have undergone massive changes over time.

It's this violent past that holds the key

to our whale shark mystery.

We're like naturalist Agatha Christies.

100 million years ago, what is now

the Yucatan Peninsula was the bottom of a warm, shallow sea.

Over millions of years, coral reefs

build up a layer of limestone almost a mile thick.

And as powerful tectonic forces stir beneath the Caribbean,

they begin to push this limestone slab up

above the water.

Rain then begins to dissolve the exposed limestone

and gradually carves out tunnels and caverns.

Stalactites begin to form like melted candles, drip by drip,

over millennia.

In some places, the roofs of these tunnels collapse,

creating the cenote entrances.

But then the Earth's climate changes dramatically.

Water levels rise, flooding the caves,

the final step in creating the underwater labyrinth we

see today.

That's not scary.

But this violent geological past has

a far wider impact, over time, affecting

the whole of the Caribbean.

Venturing deeper into the Mayan underworld,

more mysteries are revealed.

A ghostly boundary, like the surface of a shimmering lake,

but underwater.

This is the halocline, no relation to Kevin Kline,

where fresh water from above meets sea water below.

And it's a vital clue in the mystery of the whale shark

gathering, proving that the fresh water cenote network is

actually connected to the Caribbean Sea,

forming the largest underground river system on our planet.

And this unusual river system has an equally unusual effect.

On its slow journey beneath the Yucatan,

all traces of silt and particles settle out.

So by the time it reaches the ocean,

the water is crystal clear.

That's why coral reefs can thrive so close to shore.

They need clear water to survive.

That makes sense.

But it's also why, further out, the Caribbean

Sea is a blue desert, at least most of the time.

It's winter, and something amazing is about to happen.

The blue desert is suddenly alive,

sardines feeding on a sudden explosion of plankton.

Giant schools of fish race to take

advantage of this seasonal feast,

and predators are close behind them.

The largest gathering of whale sharks in the world

will soon happen right here in the clear waters off Mexico.

It should be impossible for these barren waters

to sustain such an event.

It's like the Fyre Festival.

But a sudden explosion of life is a clue to why

this place is so special.

Bonito, a small species of tuna, rise from the depths

and launch themselves on the helpless sardines.

Tuna are the sports cars of the fish world,

fast, sleek, and powerful.

They split the sardines into small groups

and annihilate them.

For the confused sardines, the only escape is up,

but the surface holds another terror.

Frigatebirds join the feast and attack from above.

Unlike most seabirds, their feathers

aren't waterproof, so they can't dive for their food.

Instead, they hover and pick off sardines one by one.

It's like me eating a Caesar salad.

But where did all this life come from in what is supposed

to be a lifeless place?

Again, the answer lies in this area's turbulent past.

20 million years ago, the same forces of geological violence

that gave birth to the Yucatan are

reshaping the whole Caribbean.

Continents collide and islands shift,

creating a narrow channel between the Yucatan

and the island of Cuba.

The waters of the Caribbean start

to funnel through this gap, creating

a powerful new current.

But most important is what happens under the surface.

Deep down, the current follows the seabed,

but as it hits the corner of the Yucatan,

it's kicked upwards, transporting nutrients from

the seabed towards the surface.

But the upwelling doesn't act alone, upwellings rarely do.

Right now, it's the strong winter winds

mixing surface waters that are the final ingredient

for this explosion of life.

And the wanderers of the open ocean

are quick to take advantage.

The sardines that have escaped the bonito and the frigatebirds

continue to feed on the upwelling,

but they're about to face the ultimate Caribbean predator.

The sailfish, the fastest animal in the ocean, unless I'm

being chased by a sailfish.

And it has company.

Their streamlined bodies can cut through the water

at 30 feet per second, and sailfish hunt as a pack.

While the pack corrals the sardines,

the first hunter approaches, but not at full speed.

Sailfish attack slowly and deliberately.

It inserts its bill into the bait ball almost gently,

then knocks a sardine out of the safety of the school,

like an Olympic fencer eating a kebab.

Then the next hunter takes its turn.

Even if they don't succeed at first,

their slashing bills wound many sardines,

making it easier for the next sailfish to catch one.

As they attack, the predators flare their signature sail

and pectoral fins to stabilize their body and strike

with better precision.

The sailfish attack relentlessly until the bait

ball is wiped out, then disappear back into the blue.

Yeah, that's not ominous at all.

Only a few sardines survive, not a great day

for the old sardine.

This intense explosion of life dies away as the winter winds

subside, and the nomads of the open ocean

move on to search for the next feast.

The waters off the Yucatan are clear and empty once more.

But inland, deep below ground in the cenote network,

other life and death dramas are playing out.

The ancient Maya who once ruled the Yucatan believed cenotes

were portals to the land of the dead.

They called this realm Xibalba, which means place of fright,

a dark and dangerous underworld ruled by death gods

and bloodthirsty predators.

Today, we are discovering the Maya were right.

There are strange and frightening creatures living

in this underwater labyrinth, and my guess is

they're pretty weird looking.

The world's largest gathering of whale sharks

occurs off the coast of Mexico, and its mysteries

are tied to secrets hidden deep in an underground maze.

Only a few animals can survive down here

in the darkness, bizarre species uniquely

suited to life in the underworld,

like the Mexican blind brotula.

This fish's ancestors found their way into the cenotes

from the sea and never found their way out.

Over time, they evolved to become specialist

dwellers of the darkness.

There's no point in being colorful in the pitch black,

so they lost all pigment from their skin,

like me with my shirt off.

And it's useless having eyes, so the cavefish lost those, too.

Now, these blind predators sense vibrations

through their thin skin and feel their way around

with two spindly pelvic fins, creeping

across the cenote floor searching for prey

like a tiny Mayan death god.

They're the apex predator down here,

the equivalent of a great white, but just four inches long.

Scientists are still discovering what

lives in the dark labyrinth beneath the Yucatan Peninsula.

It's estimated that just 10% of the underground river system

has been explored.

Every dive seems to bring incredible new discoveries.

In a handful of cenotes are wonders

seen nowhere else on Earth.

Strange formations hang from the walls

and ceiling of this chamber like college dorm showers.

The stunned divers who discovered

them called them Hell's Bells, and the name has stuck.

The alternate name was cavern churros.

The largest are over six foot long, yum.

When scientists analyzed them, they

realized something amazing--

Hell's Bells are alive.

Although they're mostly made of limestone,

they are in fact colonies of primitive bacteria.

They don't need the sun to provide energy.

They don't even need oxygen.

Instead, they feed directly on a hazy cloud

of toxic chemicals trapped in these particular cenotes.

Some scientists believe this may be what the first life on Earth

looked like billions of years ago.

Or it could be a realistic model of primitive life

on other planets.

At last, the dark underwater labyrinth

gives way to the clear, blue Caribbean Sea--

ah, that's better-- where the secrets

of the world's biggest gathering of whale sharks

are finally coming to light.

It's summer in Mexico's Yucatan Peninsula.

Tourist season is in full swing, and visitors

from all around the world are drawn

here, including giant fish.

Hundreds of whale sharks from all over the Atlantic

are converging on the same small patch of open ocean

about 20 miles offshore.

The biggest gathering of the biggest fish on Earth

is about to begin, and now we know why.

This time, it's the summer heat that energizes and intensifies

the Yucatan's upwelling.

And once again, nutrient-rich water

is being forced to the surface of the Caribbean Sea,

causing life to explode.

Bonito tuna arrive in vast numbers,

and whale sharks begin to suck in water and filter it of food.

Aw, he looks happy.

First, dozens, then hundreds, a spectacle

seen nowhere else on Earth.

But what exactly are they feeding on?

There are other whale shark gatherings around the world

where they feed on plankton in murky coastal waters,

but the biggest gathering of them

all is in the clear, blue offshore waters

of the Caribbean.

Here, hundreds of the biggest fish on Earth

are gorging on something different, something

invisible to the naked eye.

And the key are the Bonito tuna.

They're not here to feed.

The warm summer currents and the flush of nutrients

make it a perfect time and place to spawn.

Every night, the huge schools of spawning Bonito

produce billions of tiny transparent eggs.

If you could see them, they would fill the ocean.

They float to the surface in a dense layer

of nutritious food so concentrated

that the giant fish don't even have to move.

They just hang there, consuming over 300 pounds of eggs a day,

like an all you can eat caviar bar.

This is the ultimate reason for this extraordinary gathering,

a nutrient explosion that triggers

a mass tuna spawning and a feast for hungry whale sharks.

And it's all thanks to the unique geological history

of this one small area off the Yucatan Peninsula.

But there's one last twist in the story

and a surprising connection between the Yucatan,

whale sharks, and the massive cosmic disaster that

helped wipe out the dinosaurs.

The Yucatan is honeycombed with cenotes

and underground passages, but they're

not just randomly scattered.

Plotted on a map, a clear pattern emerges.

The deepest cenotes are part of a huge ring 120-miles across,

the lingering footprint of a gigantic crater and ground

zero for one of the largest cosmic catastrophes

in Earth's history.

It always comes back to things slamming into this planet.

Mexico's Yucatan peninsula is the site of the world's biggest

gathering of whale sharks.

It is also the scene of one of the largest disasters

to ever strike the earth.

And the two are connected.

It begins around 66 million years ago

when a meteor the size of Manhattan takes

aim at planet Earth.

It tears through the atmosphere at 20 times the speed

of a bullet and strikes a shallow tropical sea, blasting

a crater 15-miles deep--

yikes-- and triggering earthquakes,

volcanic eruptions, and mega tsunamis.

Do we have ways to prevent that now?

Worse is to follow.

The world burns, and the debris from the impact

blots out the sun for years, killing all plant life

and leading to the collapse of global food chains.

Sound familiar?

50% of the species on Earth become extinct.

Over the next 66 million years, the crater

becomes buried under almost a mile of sediment.

Today, the only visible sign of it on the surface

is the ring of cenotes which formed

around the edge of the crater where the limestone

was most heavily fractured.

As devastating as this mass extinction was,

it turns out the whale shark may have

reasons to be grateful for it.

Before this extraterrestrial disaster struck,

they didn't even exist.

The oceans were once home to a whole different group

of giants.

Titanic filter feeders called Pachycormus

rule the ancient seems.

Some are at least 54-feet long, the largest fish to ever exist.

Think of the size of the bagel.

When the Pachycormus ruled the seas,

the whale sharks ancestors were small bottom dwellers,

like today's carpet sharks.

But their long reign comes to a dramatic end.

When the meteor strikes and the sky

goes black, marine food chains collapse, too.

Without the sun, the plankton dies,

and there is nothing for giant filter feeders to eat.

The Pachycormus become extinct, along with 90% of the species

in Earth's oceans.

But life always finds a way.

Eventually, the sun breaks through again.

Marine plankton gradually returns to the tropical oceans,

creating a rich food source with nothing big to eat it.

The small, bottom-dwelling carpet shark

survive the mass extinction and take advantage

of this gap in the market.

One group evolves to become giant filter feeders

and takes the title of biggest fish on the planet.

Don't you want to just stick your finger in his mouth?

Yeah, I wouldn't.

OK, yeah, that-- nope.

The giants of the Caribbean ultimately owe their existence

to one of the worst days in Earth's

history, so, silver lining.

And by a cosmic coincidence, their biggest gathering

occurs almost exactly at ground zero of this mega disaster.

After weeks, or even months, of feasting,

the incredible gathering of giant fish comes to an end.

Hundreds of whale sharks begin to disperse

and head their separate ways.

But where do they go?

Considering how large and conspicuous whale sharks are,

it's surprising how little we know about them.

We know they swim thousands of miles to get to the Yucatan,

but how do they navigate?

Nobody has ever seen the giant fish mate or give birth.

Is there a secret whale shark breeding ground or singles bar?

They can dive to over a mile deep indefinitely,

so their secrets may be hidden way below us.

The planet is still full of mysteries.

I'm David Schwimmer, and this has been "Mysterious Planet."

[music playing]

Seriously, where do the whale sharks go?

Anybody?

Anybody?