Original subtitles

[distorted burbling]

[Barack Obama] Bottlenose dolphins.

[echolocation trilling, squeaking]

This pod lives in the warm shallows off Western Australia,

home to one of the largest seagrass meadows on Earth.

[quirky, plucky music playing]

[echolocation trilling]

Food here is hard to come by.

There are some fish,

but catching them is difficult,

especially the slippery and aptly named butterfish.

[echolocation crackles]

Dolphins use sound to hunt,

but the seagrass interferes with their echolocation.

[sustained trilling]

And if they get too close,

the butterfish just drop down and hide in the seagrass.

To outsmart the little fish,

the ocean's most intelligent mammals are going to have to be a lot more inventive.

[slick, groovy spy-fi music playing]

This young female is developing the skills

to turn an empty shell into something more useful.

Butterfish are curious...

and they can use shells for shelter.

Could this work?

Each shell has to be in just the right spot.

Close to the seagrass,

but out in the open.

She's impatient...

and tries to speed things along.

That was close.

[echolocation trilling]

[echolocation intensifies]

Got one.

Removing the butterfish might appear to be a problem...

but for the dolphin, it's easy.

She's figured out that if she holds the shell above the water,

the little fish will slip out.

Just like butter.

[bold, energetic music playing]

Nowhere else have dolphins been quite so clever...

and combined such complex behavior with tool use.

It's the quest to eat and not be eaten

that drives ingenuity in the tropical Indian Ocean.

[sweeping, stately music playing]

The global ocean current surges west,

carrying warm tropical seawater on what's typically a two-year journey

from Asia to Africa.

It begins by bringing life to a region of Southeast Asia

known as the Coral Triangle.

A maze of 25,000 tropical islands...

through which 15 million tons of water flow every second...

constantly refreshing the sunlit shallows

and supporting the greatest variety of marine life...

found anywhere.

[wondrous, rousing music playing]

With over 80,000 species living so closely together,

coral reefs don't get any more competitive or dangerous than these.

[music swells, trails off]

But food is surprisingly scarce.

And with millions of hungry mouths to feed,

every reef resident is on someone else's menu.

[delicate, mysterious music playing]

Keeping your babies safe is an around-the-clock challenge.

One expectant mother is especially inventive

as she tries to protect her brood.

The peacock mantis shrimp.

[eccentric music playing]

She's a reef engineer...

with an unrivaled mind map of her neighborhood.

Her passion is for finding fragments of dead coral.

She logs the position of each one in her memory,

in case she needs it.

And she'll have to be quick,

because the reef's recyclers are busy

turning all the dead coral they can find...

into sand.

This piece looks just right.

But why is she so picky?

Because she knows exactly what shape of coral she needs next

in her construction project.

It's thought she may even remember

the structure of the pieces she found earlier...

slotting them into a retaining wall so each one fits perfectly with the next.

Might be useful.

Wrong shape.

With her masonry skills,

she's built a fortified nest chamber

where she can protect her 10,000 eggs until they hatch.

[delicate, wondrous music playing]

And if you want to get in,

you'll have to get past Mom first.

No chance.

Thanks to her engineering prowess,

this clever mom has solved the biggest problem on the reef.

How to raise the next generation in safety...

amidst all these hungry mouths.

[music trails off]

But there are other ways to make sure your offspring get the best start in life.

Bumphead parrotfish don't hide their eggs.

They have another way to outwit the reef's egg eaters.

But before they spawn,

mating rights must be decided 100 feet below the surface.

[somber, uneasy music playing]

Because there's a perfect time and place to mate.

The place is the drop-off,

where the tide and current flow away from the reef,

helping sweep eggs to safety.

The time? When the tide reaches its peak.

The contestants each search for an area where the current will be strongest.

The largest claims the best site.

[fronds rustling unsettlingly]

Whoever can hold that spot will be chosen by the most females.

And the eggs he fertilizes will be swept furthest away from the reef and predators.

But how is he going to attract females still high up on the reef...

without leaving his position?

He can send off smaller rivals with a flick of his tail.

But not this guy.

One hundred pounds of bruising bumphead.

[harsh, menacing music playing]

A rival with his eyes on the same mating spot.

He's not intimidated.

There's only one thing to do.

For the first time,

we have a ring-side seat at this heavyweight battle.

[collision booms]

Their collision is so violent that water trapped between their bumps is vaporized.

Their head guards are cupped to focus the force of the collision.

[thud booms]

It creates bubbles of steam that, at this depth, implode...

[collision reverberates]

...sending shockwaves across the reef,

announcing to the audience that the fight is on.

Their heads aren't their only weapon.

That coral-crunching beak now comes into play.

[thud booms]

Their technique is crash and slash.

Slicing scales off each other.

As the current builds,

more females arrive.

[music intensifies]

At last, they've drawn a crowd.

Final round.

[thud booms]

As the current peaks, there's nothing to separate them.

[forceful '80s rock music playing]

Exactly what they both wanted.

Because only with such a well-matched adversary

can they create the loudest sound on the reef.

[thud booms]

[booms echoing]

Signaling they're the strongest.

After this bout, they're both winners.

[music trails off]

Their performance has won them dozens of eager admirers.

Now, they're ready to spawn...

in the right place...

and at the right time...

for the tide and current to work their magic...

[bright, rousing music playing]

...and transport the next generation of bumpheads to safety,

far away from the reef.

[music trails off]

The global ocean current winds its way between the islands of Southeast Asia.

As it goes, it collects evidence

of the throwaway lifestyle of the world above,

before sweeping off the coral reefs

and into sandy wastelands.

[measured, downcast music playing]

Up to two million tons of plastic waste enter the Indian Ocean every year.

Some of the highest levels of plastic pollution found in any ocean.

Each piece will take hundreds of years to break down.

Eventually becoming microplastics...

damaging ocean health

and even making their way into our food chain.

But there is one resident here who tries her best to make the most of this mess.

[music subsides]

The veined octopus.

She's looking for a place from which she can ambush her prey.

[delicate, quirky music playing]

And items discarded by humans offer a wealth of possibilities.

As they say, one person's trash is another's treasure.

A bottle could be a perfect hideout.

It just needs a bit of rearranging.

Here we go.

Whoa.

Let's try that again.

Got it.

There we go.

This will work. Much more roomy.

Yeah.

But if she can see out...

maybe her prey can see in.

Hmm, not gonna work.

Can nature offer a better solution?

With two of these...

and a little digging...

enough room to peek out...

and for her siphon to breathe...

the octopus can disguise herself as a clam.

Perfect for setting her ambush.

[suspenseful music playing]

[eccentric music playing]

She'll just eat the flesh and discard the leftovers.

But leaving scraps lying around is not a good idea.

It attracts scavengers.

Fish that will scare away the crabs.

She needs to work out how to make them leave without blowing her cover.

[pellet zings]

[old-timey Western stand-off music playing]

So she turns her siphon into a gun.

Reload.

Until now, we didn't know an octopus

could arm itself like this.

Genius.

Perhaps the only creature in all the oceans

clever enough to use a projectile as a weapon.

[stand-off music subsides]

With the coast clear, she can return to the hunt.

[eccentric music playing]

And she's quick to learn from her mistakes.

This time she takes out her trash...

and tips it far enough away that her hideout remains a secret.

At least someone here has learned how to take care of their waste.

[lilting, melodious music playing]

The global ocean current leaves the Coral Triangle and flows west,

into the open ocean.

But it carries little life, as food here is sparse.

It's a marine desert.

And yet, it hides giants.

[serene, understated music building]

Great travelers of the current,

who are just waking up.

Sperm whales.

[echolocation trilling]

A tight-knit family group...

[delicate, enchanting music playing]

...with a baby just weeks old, but growing fast.

He's hungry and persistent.

[echolocation ticking]

To produce the rich milk he needs,

Mom must eat hundreds of pounds of seafood every day.

[drawn-out lowing]

And that means leaving these barren, sunlit waters and diving into the abyss.

However much he might want to,

Baby can't tag along on her 45-minute dives.

It might be three years

until he is able to hold his breath long enough to join her.

But he can hear her.

[echolocation trilling echoes]

Hunting for giant squid over a mile below.

[sustained, steady lowing]

He knows he hasn't been abandoned.

His sisters and aunties gather around, all taking the chance to play with him.

[overlapping, excited trilling]

Smothering him with love and affection.

[enchanting music continues]

Sperm whales have the largest brains on the planet,

six times the size of ours.

And it's thought with three times more devoted to empathy.

It's their close family bonds and emotional intelligence

that helps sperm whales survive in this wilderness.

Finally, full of squid, Mom returns...

with rich milk for her baby.

On her way up, she has stirred up the water column

and brought deep-sea nutrients to the surface,

enriching this marine desert and the ocean current

as it continues its journey across the central Indian Ocean.

[wind rushing]

[tranquil, contemplative music playing]

Past tiny specks of land...

the Maldives.

Chagos.

The Seychelles.

Each an oasis in the marine desert.

A magnet for sea life.

And holding, for some, the promise of plunder.

The giant trevally, or GT for short.

At 30 pounds, he's still a youngster.

He's a hunter and might one day become one of the atoll's apex predators.

[adventurous, spirited music playing]

His arrival has coincided with an important event

in the island's calendar.

[flock squawking]

The week the ground-nesting terns learn to fly.

The chicks need to build their stamina,

but they tire quickly and need frequent rest.

For a small GT who needs to grow big and strong...

it's an opportunity to change his diet.

At first, he's cautious.

He's lost out.

No match for these ferocious adults.

[tense, aggressive music playing]

At up to 170 pounds,

mature GTs can reach speeds of 40 miles an hour.

And they've honed their technique.

[music intensifies]

Some of the adults have been returning for this annual feast for over a decade.

He must learn to hunt just like them.

To approach from behind if he can, then accelerate.

[flock cawing]

Here's one.

[tense music continuing]

The chick is tired.

It's the best chance he'll get.

[music sputters awkwardly]

[music trails off]

But birds are hard to catch.

Especially in flight.

Within the week, the chicks behave more like the adults...

[flock squawking]

...able to stay in the air longer...

and rarely sitting on the water anymore.

He now needs to assess airspeed and altitude.

[tense music playing]

He can't see well above water.

So, it's all about perfecting his trajectory.

[spirited music rising]

[tern squawks]

[determined, spirited music intensifies]

[music subsides]

This season, the more experienced GTs get there first.

It may be years until he masters the skills

and grows big enough to join the ranks of the truly giant trevallies.

[rousing, stately music playing]

After two years flowing through the emptiness of the big blue,

the global ocean current collides headlong with East Africa,

mixing with food-rich coastal waters.

And here, the current splits.

The majority heads south.

But during the summer months,

some of it flows north towards the Arabian Gulf.

Where, under the tropical sun...

the rapid rates of evaporation have created

one of the saltiest seas in any ocean.

[intriguing music playing]

It's home to a rarely seen inhabitant.

Once mistaken for a mermaid.

Today, dugongs are still creatures of mystery.

These gentle sea cows were once common along the coasts of the Indian Ocean,

from Australia to Africa.

In some areas, they've been hunted almost to extinction.

[delicate, gentle music playing]

Their quest for lush grazing ties them to seagrass meadows.

But it's recently been discovered

that dugongs will make long-distance migrations

to take part in great gatherings.

Exactly why they do this is a mystery.

Today, it's a rare privilege to see a sight like this.

One that may only be possible because of the refreshing influence of the current.

Around the world, seagrass meadows, and the animals that depend on them,

are declining as they are threatened by rising sea temperatures.

[distorted whooshing]

This building heat causes more and more water

to evaporate from the current,

intensifying the greatest weather system on our planet.

[wind whipping jarringly]

The monsoon.

[thunder rolling]

Every year, it sweeps through the northern Indian Ocean.

And today, it's more powerful and unpredictable than ever.

The monsoon has always fed the Indian Ocean

with nutrients from the land.

But these days, it brings something much less welcome.

Pollution.

[haunting, hushed music playing]

Trillions of pieces of plastic end up here every year.

Manta rays are wanderers

that have been riding the global ocean current

for more than 20 million years.

But today, as they filter feed on plankton,

they can't avoid ingesting these plastics.

And for every piece they can see,

there's a blizzard of plankton-sized particles they cannot.

One manta ray caught in discarded fishing gear

has no way to remove it.

The plastic line will eventually cut through its flesh.

Ingesting plastics or becoming entangled in them

is a modern risk for these ancient rays.

But it doesn't have to be this way.

We may have damaged our oceans,

but if we choose to manage our waste better,

we can help heal them too.

[delicate, pensive music playing]

Off southern Mozambique,

there's a special place where manta rays come

to seek treatment for their infected wounds.

And they're not alone.

Many species of ray seek help here,

including a rare wedgefish, a ray we've barely gotten to know,

but that's already critically endangered,

targeted by humans for its high-value fins.

But it's the mantas that need the most help healing.

This female is a shark attack survivor.

The bite damage to her wings risks infection.

So, she's visiting her infirmary.

The medical team goes to work.

They remove infected tissue,

which promotes new growth.

Her injuries are their food.

Hundreds of manta rays ride the current here for emergency care.

But there are risks.

Bull sharks patrol these waters.

[menacing undercurrent plays]

So, these intelligent rays watch each other's backs...

allowing all of them to get the essential care they need.

With the right treatment, their bodies have incredible powers of regeneration.

[bright, hopeful music playing]

These patients have been given a second chance...

thanks to the care they've received at this special sanctuary.

A stronghold for so many rays,

the gentle wanderers of the global ocean current.

Now, heading south, the great current picks up speed and a new name,

becoming the Agulhas Current,

carrying equatorial heat toward the tip of Africa,

where it begins to mix with the cooler, richer waters of the Atlantic Ocean.

[dramatic, majestic music playing]

Ideal conditions for the great white shark.

[measured, foreboding music playing]

It's one of the few warm-blooded sharks,

which improves muscle performance and sharpens its mind.

To survive here, in waters ruled by the great whites,

you have to be smarter.

[seal brays]

A young seal pup, just eight weeks old.

Born into a colony of Cape fur seals.

She'll need to learn how to survive with great whites on her doorstep.

It's time for this new student to enroll at seal school...

where her classmates are already being taught

the most important lessons of all.

Rule one, keep away from the sharp end.

[taut, suspenseful music playing]

Rule two, never let your guard down.

[seal brays in alarm]

Wise seals don't get caught napping.

[menacing music intensifies]

You can learn from others' mistakes.

The most important lesson of all, though, is to work together.

[riveting music playing]

Their ability to act as one

makes a group of fur seals stronger than any individual shark.

Against the mob...

even a great white will back down.

[brays confidently]

But the real battle of wits is yet to come.

[huffs]

A southern elephant seal.

A raider from Antarctic waters,

looking for new opportunities.

At first, the fur seals don't know what to make of this stranger.

He looks like a seal...

but he's even bigger than the shark.

[sudden, jarring music playing]

[guttural bellow]

And, to their surprise, just as deadly.

[threatening snuff]

The pup knows that land is a fur seal's refuge.

[guttural bellowing]

[foreboding music intensifies]

But this predator doesn't stop at the water's edge.

[distressed mewling]

[bellowing]

She's cornered.

[elephant seal snarls]

[rattly, jarring growling]

But in this group, she's never alone.

[brave, resolute music building]

She just has to stand her ground...

until backup arrives.

[resolute music intensifying]

[mewls]

[brays]

[mewls]

They've already sent off a great white.

Together, they'll send this elephant seal packing too.

It's their strength in numbers and collective intelligence

that has allowed these fur seals to survive here at the Cape.

And the effort is worth it...

because once a year,

as the warmer Indian Ocean current

traps the cooler Atlantic waters against the coast,

it forces together shoals of fish in vast numbers.

It's known locally as the sardine run.

Hundreds of millions of sardines form shoals up to five miles long.

Right on the fur seal's doorstep.

[bold, sweeping music playing]

But they won't have it to themselves for long.

Here come plunge-diving gannets.

And up to 20,000 dolphins.

[echolocation trills]

Along with thousands of sharks.

And even...

great whales.

It's an unrivaled bounty, only made possible by the current...

transforming what was a marine desert...

into one of the greatest feasts in all our oceans,

right at the very end of its journey through the Indian Ocean.

Next, the global ocean current continues its journey.

Flowing from the Indian to the Atlantic Ocean,

where its fast-heating waters are growing in power.

[majestic, soaring music playing]

The Atlantic is getting wilder than ever.

[brays]

And if you run into trouble, you'll need a team to help you out.

[music subsides]

[sparkling, quirky outro music playing]