Original subtitles

- [dolphins clicking] - [water lapping]

[Hiddleston] Our shallow seas cover 10% of the ocean.

A tapestry of coral reefs,

sandy flats and seagrass meadows,

long thought to be silent worlds.

But in recent times, new underwater microphones have revealed

that these waters are alive with extraordinary noises.

- [cooing] - [bubbling]

[honking]

A treasure trove of sounds

that is changing our understanding of life beneath the waves.

[humming]

And perhaps the strangest sound

can be heard off Australia's Great Barrier Reef.

The largest reef on the planet.

Here, each winter,

mysterious noises come out of the blue.

[whale calling]

Once thought to be the sonar of submarines...

we've recently learned that the truth is even stranger.

[calling continues]

For this is the song of the dwarf minke whale.

They spend most of their lives alone, way out in the open ocean.

But for just a few weeks each year,

they travel to the ribbon reefs off the northern edge of Queensland.

It's thought that whales use the coral reef

as a kind of amphitheater

to broadcast their otherworldly calls...

[calling]

...named by scientists as the Star Wars vocalization.

It's created by pumping air between their throat

and their thousand-liter lungs...

[calling continues]

...producing two harmonies simultaneously.

A high melody.

[chirps]

- And some deep base. - [rumbles]

In shallow water,

sound travels five times faster than in air.

[calling continues]

So, when they call,

it soon attracts the attention of others.

[trilling, calling]

Incredibly, this is the only place in the world

where these whales come together and sing like this.

It could be a chance for males to show off to each other...

[trilling, calling continues]

...or perhaps impress the females.

Since scientists realized that it was whales

producing these alien-like calls,

we've discovered a whole new world of sound in the shallow seas.

[birds chirping]

And the noisiest place of all...

is on the reef itself.

[cooing]

[clicking]

Here, creatures click, pop, and honk

to defend territories and attract mates.

Only passing predators, like reef sharks, temporarily silence things.

- Low booms, recorded for the first time... - [booming]

...are thought to be love songs of grouper fish.

And schooling jacks don't just hear noise,

they feel it too,

using vibrations to stay in formation.

[crackling]

Healthy reefs are so loud, they can be detected from ten miles away.

[chirping, honking]

In this coral cacophony,

making yourself heard is a constant challenge.

[honking continues]

- So, one little fish... - [trills]

...has recently started to make a brand-new sound.

The Ambon damselfish.

Just four inches long, they are the Romeos of the reef.

Males spend much of their time sprucing up their bachelor pads

in a bid to impress the females.

For this young male, cleaning up is a bit of a struggle.

But when everything's just right,

it's time to find a date.

On such a busy reef,

there can be hundreds of species clicking and popping.

[clicking, popping]

So, damselfish do something different.

They sing.

We can only tune in to their delicate warbles

using specialist microphones.

[humming]

Most remarkably,

this was heard for the first time just eight years ago.

Scientists believe that one male began to sing this short melody

and then other males copied him...

making it perhaps nature's newest love song.

[humming continues]

This underwater ballad soon attracts a female's attention.

Clearly a singing fish is quite a catch.

Across the planet,

noisy coral reefs make up less than 10% of the shallow seas.

Most of the seafloor is expansive sand flats.

In the quiet waters of the Bahamas,

we're now learning that animals use sound in evermore sophisticated ways.

Perhaps none more so than the Atlantic spotted dolphin.

[squeaking, clicking]

For these intelligent creatures,

the greatest challenge in this seemingly barren land

is finding fish to eat.

But we now know that they do this

using an astonishing sonic superpower.

Like all dolphins, they hunt using echolocation.

[squeaking, clicking continues]

But in shallow waters,

instead of using this skill to find food in front of them,

they use it to search for fish completely hidden from sight.

By forcing air around their nasal passages,

they create high intensity clicks...

[clicking]

...which are so powerful that they penetrate through the sand,

bouncing off objects beneath.

[clicking continues]

This allows Atlantic spotted dolphins to scan the seafloor like metal detectors.

When prey is detected,

they ramp their clicks up to over 400 per second.

[buzzing]

Known as "the buzz,"

it's so forceful that it can stun the fish,

making them easier to catch...

as long as you don't let go.

[clicking, buzzing continues]

Success at last.

[whistling, clicking]

The dolphins' clicks make them the masters of acoustic hunting in the shallow seas.

But on other tropical sandbanks in Southeast Asia...

one little fish also uses clicks,

but for a very different purpose.

Saddleback clown fish.

Small fish with big attitudes.

[clicking]

This raucous group of around a dozen

has set up home on a sea anemone

the size of a dinner plate.

Unlike their cousins on the reef...

[clicking continues]

...these fish have almost nowhere to hide from danger.

So, the colony relies on sound.

The smaller fish spend their days playing rowdy games of chase.

They're ruled over by the only female.

The biggest and loudest fish.

[clicking]

By knocking her teeth together,

she produces powerful clicks

to keep the colony in order.

Because she needs them to work with her to protect her precious offspring.

[clicking]

Hundreds of little clown fish eggs, each just the size of a grain of rice.

The largest male softly calls to them,

a behavior that has never been recorded before.

[clicking continues]

Incredibly, scientists believe that the baby fish

may sense this sound inside their egg sacs.

So, when they hatch,

they can already recognize the calls of their own species.

Less than 1% will make it to adulthood

because they're the perfect sized snack

for hungry predators who lurk beneath the sand.

A swimming crab.

An aggressive hunter with a particular taste for fish eggs.

[clicking]

With the intruder spotted,

the clown fish alarm call to alert the colony to the threat.

And the female begins an acoustic attack.

[popping]

Her loud pops, a warning to leave the eggs alone.

[clicking, popping continues]

Combined, the clown fish form a noisy mob.

Finally, the biggest male gets physical,

pushing with all his might.

And the crab is sent scuttling.

By using their voices, and a little brute force,

these feisty clown fish work together to protect the next generation.

[birds chirping]

But in shallow waters,

it's not just animals that make unusual noises.

Off the coast of Florida...

plants make sounds too.

Here, vast seagrass meadows turn the ocean an emerald green.

As sunlight penetrates the water, they start to fizz.

[fizzing]

Seagrasses use light and carbon dioxide to make their food,

and in doing so produce tiny bubbles of oxygen.

Slowed down over a hundred times...

each one creates the faintest whooshing sound...

[whooshes, bubbling]

...as it peels away from the leaf.

Billions of these bubbles bursting free...

build to a sizzling soundscape.

[fizzing]

These meadows absorb carbon dioxide 35 times faster than the rain forest...

[fizzing continues]

...and produce so much oxygen, these shallow seas are vital

in the battle against climate change.

[bubbling]

For one precious gentle giant,

the sound of fizzing vegetation must be like a dinner bell.

[snuffling]

The West Indian manatee.

A distant relative of the elephant,

there are less than 7000 individuals left in the wild.

From an animal that weighs half a ton,

you might expect to hear great grunts or bellows.

But in fact, manatees make a surprising sound.

[squeaking]

It might not sound like much,

but new research shows that these high-pitched squeaks

travel up to 800 feet in quiet, shallow waters.

For mothers and calves, this is crucial for keeping together.

Each winter, temperatures out at sea plummet so low,

the little ones can perish in the cold.

So, they must travel six miles inland, through rivers,

to reach the safety of warm water springs.

[squeaking]

An annual manatee migration.

[squeaking]

When the playful calf wanders off...

Mum calls her back in line.

[squeaking continues]

The pair constantly chatter

so they can stick together on their journey.

But in recent times,

the soundscape of the manatees' ancient migration route

has started to change.

[birds squawking]

The human population in Florida has increased dramatically.

There are now a hundred and thirty vessels for every manatee.

In shallow waters,

propeller sounds can disorientate the manatees,

drowning out their soft squeaks...

making it harder for mums and calves to keep in touch.

It takes two days of swimming before the sound of boats starts to fade.

[squeaking]

And in the distance, there are some familiar chirps.

[manatees chirping, squeaking]

This mum and calf aren't the only ones to have made it.

Manatees from far and wide have traveled from the dangerously cold ocean

to this warm spring.

[chirping, squeaking continues]

By using squeaks to keep in touch...

these threatened mammals can make it to this lifesaving refuge.

It's just one of the many new discoveries

that are changing our understanding of the importance of sound

in these so-called silent waters.

To capture the soundscape of the coral reef

in more detail than ever before...

[clicks]

...the Earthsounds team traveled to Raja Ampat

in the heart of Southeast Asia.

[crew member] The underwater world is famously called

the Silent World by Jacques Cousteau, but it's really far from it.

There's all sorts of popping and clicks and almost whistles that you hear.

So, there's a real cacophony of sounds down there.

[honking]

[cooing]

[Hiddleston] With the reef so busy,

working out exactly who is calling can be tricky.

So, the team enlisted the help of marine biologist, Isla Hely.

On tropical reefs, fish chat to each other for a myriad of reasons.

We've known this for a... a long time,

but we've been limited by the technologies that we have to capture that sound.

[Hiddleston] Isla hopes to be able to decipher some of this reef chatter

using a sound recording device built specially for the series.

So, this is our diver-operated recording apparatus.

We like to call her DORA.

So, there's the microphone on top, which is a hydrophone,

but then you also have a camera

so you can match what's making the noise with the visuals.

[Hiddleston] DORA enables Isla to record the sound of the entire reef

or point and focus on individual species.

- [clicking] - [crackling]

And with its removable head,

Isla can suspend the hydrophone away from her,

allowing her to capture close proximity recordings

of reef residents who like to hide away.

[Hely] We've collected a whole cohort of sounds.

There's some base-y, low frequency, boing-y sounds...

[booming]

...and we believe those to be grouper.

Um, we have the quite sweet weeping sound of the Ambon damselfish.

[humming]

But there have been some bizarre sounds that we've been picking up.

[buzzing, throbbing]

There could be a cryptic species that's just hidden in a cave

that we just haven't been able to see.

[Hiddleston] Armed with DORA,

Isla has been able to capture individual fish conversations.

And uncover sounds that have never been recorded before.

- [crackling] - [clicking, chirping]

But whilst reefs here may be thriving and full of sound...

[cooing]

...that isn't the case across the world.

Warming oceans mean 90% of live coral reefs

are expected to disappear by 2050.

[Hely] I'm increasingly seeing more and more degraded reefs

due to climate change.

And when you listen, it's quite haunting actually

because all that sound that creates the cacophony of the reef

is missing.

[Hiddleston] But a silent reef isn't necessarily a dead one.

Isla hopes that sound could hold the key to conserving them.

Recent research has shown that when you play back sounds

of healthy reefs within a degraded reef,

you can actually bring in both fish larvae and coral larvae,

and all the settlers that would help to restore these ecosystems,

potentially to the beautiful reefs that they once were.

[Hiddleston] Through new scientific breakthroughs,

we're only just uncovering how crucial sound is

in these shallow seas.

[clicking]

And just how noisy healthy reefs really are.

Every recording is a joy to listen to.

And this is what reefs should be like around the world.

- [clicking, cooing] - [crackling]