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?