Earth's frozen poles.
Magical ice worlds where temperatures can drop to minus 120 degrees Fahrenheit.
In our planet's coldest places,
sound travels slower than anywhere else.
And animals have evolved ingenious ways to make themselves heard
in this sub-zero soundscape.
For one icon of the Antarctic...
sound is crucial to keeping their families together.
The emperor penguin.
Each year up to 20,000 of these noisy birds
gather on the ice to raise their young.
And for parents,
this makes keeping track of their little ones
a seemingly impossible challenge.
The thousands of chicks all look the same.
Even their parents can't tell them apart by sight alone.
So, the key to finding their young all comes down to sound.
This hungry youngster has been waiting three days
for his mom to return from the ocean, where she's been catching fish.
While she's away,
he tries to convince other adults to feed him.
Like all chicks, he has a unique whistle.
It's how his parents identify him.
But it also lets other adults know he's not their offspring...
and he's soon sent packing.
He needs his mom, and fast.
Thankfully, he's in luck.
The parents are on their way, each with a belly full of fish.
But to feed their youngsters, first they must find them.
When searching,
penguin parents produce what's known as the "two voices" vocalization.
With new technology,
we're able to separate these voices, revealing two sounds.
Combined...
they form a unique sonic signature.
This cuts through the cacophony of the colony.
If mom's chick is within 50 feet,
he'll recognize her call...
and whistle in reply.
But it's unlikely he'll hear her first time.
So, this mom sets off through the crowd...
calling every few minutes...
and waiting for a response.
This time, nothing.
He must be here somewhere.
She will spend hours searching.
Finally,
mom's two-voices call gets the reply she's been hoping for...
her chick's signature whistle.
After days apart, the pair are reunited...
and the youngster gets his long-awaited meal.
A heartwarming reunion...
in the coldest place on Earth.
Life above the ice is only half the story.
There's a whole different soundscape beneath.
In summer months, melting glaciers fizz...
as bubbles of air escape.
Alien-like calls ricochet off the underside of the ice.
Sound here travels further than in warmer waters.
Perfect conditions for beluga whales
to keep in touch with their birdlike calls.
But remarkably,
some polar animals have mastered both
these icy sound worlds.
The Atlantic walrus.
When hauled out on the ice,
they produce unsavory sounds,
some best left unnamed.
But the beneath the water, they create the most bizarre courtship calls.
And with the breeding season approaching...
it's time to get some practice.
When you weigh as much as a small car,
it's easy to forget you're on thin ice.
Even slipping into the sea can be a bit of a challenge.
But once in,
these ungainly animals become the most amazing vocalists.
Specialist underwater microphones now allow us
to tune in to their otherworldly calls.
A collection of the weirdest sounds, from hammering...
to metallic pulses.
And the strangest of all: the gong.
These mechanical noises are created by moving air
around vast buoyancy sacs deep in their throats.
This communal display has rarely been seen before.
It's believed to be juvenile males practicing their chat-up lines.
And they need to build up their endurance,
as walrus courtship is a lengthy affair.
Scientists have recorded males singing for a staggering 65 hours.
Making their love songs the longest-- and perhaps strangest--
of all mammals.
Whilst walrus want to make themselves heard,
for one legendary Arctic resident,
the key to success comes down to staying silent.
The polar bear.
This thousand-pound predator roams the high Arctic
on a constant quest for food.
When hunting,
their sense of smell first leads them towards their prey.
But research shows,
as they close in, the hunter switches to sound.
With enlarged eardrums,
they tune in to low frequency noises.
In cold, dense air, sound travels slower.
But strangely,
bouncing between the ice and a layer of warm air above...
noises also carry further.
And the polar bear uses this quirk of sub-zero sound
to listen out for one thing above all others.
Like the walrus,
male bearded seals perform extraordinary underwater love songs.
Females listen in from above.
But they're not the only ones tuning in.
Even with padded feet,
polar bears are too heavy to conceal their footsteps.
So, the bear has an ingenious way of using the soundscape to hunt.
Moving under the cover of lapping waves...
and creaking ice...
a technique called aquatic stalking...
enables him to get closer.
But the seal's sharp ears put her on high alert.
This time, the predator comes up empty-handed.
Polar bears only need to make a kill once every ten days to remain healthy.
So, he'll be back to try again.
In the polar regions,
it's not only the animals that create unusual sounds.
Microphones buried deep in the snowpack
allow us to eavesdrop on humming ice shelves...
as they vibrate in powerful winds.
And icebergs whistle and creak...
as they collide with sea ice.
But perhaps the strangest sub-zero sound of all...
takes place after dark.
The Northern Lights.
One of the planet's greatest visual displays.
But it's recently been discovered
that the Earth's ultimate light show produces secretive sound signals too.
These eerie whistles are usually inaudible to the human ear,
but can be picked up by long wave radio receivers.
Occurring up to 400 miles overhead,
the signals are produced
as charged particles from the sun collide with our atmosphere.
The mesmerizing song of the polar night.
From celestial symphonies,
to the most heartwarming of all Arctic sounds.
Each spring, love is in the air
as thousands of eider ducks descend on the sea ice
to breed.
Males sport unmistakable black-and-white plumage
with a green flash,
while females are a more understated brown.
With so many eager rivals,
males go all out to impress
with their rather charming mating calls.
Creating the sound requires a strong, highly flexible neck.
In fact,
male neck muscles are 50% thicker than those of females.
The call may sound unusual to us,
but for highly competitive males...
the noise triggers a chorus of rivals.
Should this fail to get the female's attention,
the Romeos have another seductive trick to play.
Flapping wings is one of a dozen eider duck courting rituals.
Duck dad dancing.
If a female accepts,
she does so with a simple snort.
The quirky calls have paired another couple.
It's a date.
Just one of the surprising ways animals use sub-zero sound
to thrive at the Earth's frozen poles.
To capture the sounds of the polar regions in more detail than ever before,
the Earthsounds team underwent the 10,000-mile journey to Antarctica.
They were to spend a month at a research base called Neumeyer Station III.
Day one.
The team strikes out in search of emperor penguins.
When you first arrive at the penguin colony,
the first thing that hits you is the sound of it.
Thousands of birds all calling at the same time.
Amidst this cacophony...
the challenge is to get a clean recording of a hungry chick's signature whistle.
There's thousands of chicks and adults, all calling together.
So, the difficulty is going to be picking out an individual call.
But on a continent where humans are something of a rarity,
the subjects turn out to be most accommodating.
We're meant to be staying 30 meters from the colony,
and five meters from any penguins.
But this inquisitive chick has come forward.
So, it might be a bit easier to get the sound than we thought.
Well, that was straightforward.
The next sound the team wants to record is underwater.
And their luck continues, with another obliging performer.
That's a Weddell seal song.
Moments later, the source of the incredible calls...
makes an appearance.
That's a mom and pup.
Clearly we've come to occupy their breathing hole,
and they just wanted to check us out.
It's so incredible that in Antarctica, the wildlife comes straight to you.
But the team's run of good luck is about to end abruptly...
as conditions take a turn.
The weather has come in pretty quickly.
There's a whole bunch of snow around us, and the visibility has got a lot worse.
So, time to go back to base.
Antarctica is the windiest place on earth.
And in recent years,
our warming climate has made the winds even fiercer.
Gales here have been known to reach 200 miles an hour.
The crew hunker down.
Three days later, the weather finally clears.
And the team gets recording again.
Starting to build a library of Antarctica's more unexpected noises.
I've really enjoyed some of the smaller sounds.
The sound of penguins' feet on the ice as their claws crunch through the snow.
The incredible underwater hubbub of air bubbles
as the birds streak up to the surface.
Then there's a sound like a squeaky toy as they jump out of the water.
It sounds like the air is leaving their body.
After four weeks on the ice,
the team have captured the Antarctic soundscape
closer than ever before.
A unique insight into the magical world
of sub-zero sound.