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Earth's frozen poles.

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Magical ice worlds where temperatures
can drop to minus 120 degrees Fahrenheit.

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{\an8}In our planet's coldest places,

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{\an8}sound travels slower than anywhere else.

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And animals have evolved ingenious ways
to make themselves heard

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in this sub-zero soundscape.

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For one icon of the Antarctic...

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sound is crucial
to keeping their families together.

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The emperor penguin.

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Each year up to 20,000
of these noisy birds

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gather on the ice to raise their young.

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And for parents,

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this makes keeping track
of their little ones

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a seemingly impossible challenge.

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The thousands of chicks all look the same.

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Even their parents can't tell them apart
by sight alone.

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So, the key to finding their young
all comes down to sound.

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This hungry youngster
has been waiting three days

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for his mom to return from the ocean,
where she's been catching fish.

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While she's away,

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he tries to convince
other adults to feed him.

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Like all chicks, he has a unique whistle.

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It's how his parents identify him.

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But it also lets other adults know
he's not their offspring...

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and he's soon sent packing.

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He needs his mom, and fast.

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Thankfully, he's in luck.

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The parents are on their way,
each with a belly full of fish.

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But to feed their youngsters,
first they must find them.

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When searching,

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penguin parents produce what's known
as the "two voices" vocalization.

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With new technology,

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we're able to separate these voices,
revealing two sounds.

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Combined...

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they form a unique sonic signature.

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This cuts through the cacophony
of the colony.

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If mom's chick is within 50 feet,

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he'll recognize her call...

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and whistle in reply.

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But it's unlikely
he'll hear her first time.

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So, this mom sets off through the crowd...

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calling every few minutes...

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and waiting for a response.

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This time, nothing.

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He must be here somewhere.

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She will spend hours searching.

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Finally,

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mom's two-voices call
gets the reply she's been hoping for...

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her chick's signature whistle.

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After days apart, the pair are reunited...

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and the youngster
gets his long-awaited meal.

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A heartwarming reunion...

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in the coldest place on Earth.

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Life above the ice is only half the story.

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There's a whole different soundscape
beneath.

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In summer months, melting glaciers fizz...

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as bubbles of air escape.

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Alien-like calls ricochet
off the underside of the ice.

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Sound here travels further
than in warmer waters.

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Perfect conditions for beluga whales

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to keep in touch
with their birdlike calls.

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But remarkably,

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some polar animals have mastered both

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these icy sound worlds.

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The Atlantic walrus.

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When hauled out on the ice,

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they produce unsavory sounds,

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some best left unnamed.

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But the beneath the water, they create
the most bizarre courtship calls.

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And with the breeding season approaching...

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it's time to get some practice.

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When you weigh as much as a small car,

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it's easy to forget you're on thin ice.

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Even slipping into the sea
can be a bit of a challenge.

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But once in,

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these ungainly animals
become the most amazing vocalists.

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Specialist underwater microphones
now allow us

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to tune in to their otherworldly calls.

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A collection of the weirdest sounds,
from hammering...

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to metallic pulses.

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And the strangest of all: the gong.

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These mechanical noises
are created by moving air

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around vast buoyancy sacs
deep in their throats.

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This communal display
has rarely been seen before.

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It's believed to be juvenile males
practicing their chat-up lines.

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And they need to build up their endurance,

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as walrus courtship is a lengthy affair.

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Scientists have recorded males singing
for a staggering 65 hours.

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Making their love songs the longest--
and perhaps strangest--

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of all mammals.

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Whilst walrus
want to make themselves heard,

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for one legendary Arctic resident,

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the key to success
comes down to staying silent.

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The polar bear.

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This thousand-pound predator
roams the high Arctic

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on a constant quest for food.

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When hunting,

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their sense of smell
first leads them towards their prey.

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But research shows,

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as they close in,
the hunter switches to sound.

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With enlarged eardrums,

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they tune in to low frequency noises.

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In cold, dense air, sound travels slower.

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But strangely,

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bouncing between the ice
and a layer of warm air above...

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noises also carry further.

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And the polar bear uses this quirk
of sub-zero sound

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to listen out for one thing
above all others.

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Like the walrus,

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male bearded seals perform extraordinary
underwater love songs.

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Females listen in from above.

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But they're not the only ones tuning in.

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Even with padded feet,

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polar bears are too heavy
to conceal their footsteps.

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So, the bear has an ingenious way
of using the soundscape to hunt.

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Moving under the cover of lapping waves...

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and creaking ice...

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a technique called aquatic stalking...

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enables him to get closer.

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But the seal's sharp ears
put her on high alert.

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This time,
the predator comes up empty-handed.

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Polar bears only need to make a kill
once every ten days to remain healthy.

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So, he'll be back to try again.

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In the polar regions,

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it's not only the animals
that create unusual sounds.

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Microphones buried deep in the snowpack

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allow us to eavesdrop
on humming ice shelves...

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as they vibrate in powerful winds.

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And icebergs whistle and creak...

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as they collide with sea ice.

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But perhaps the strangest sub-zero sound
of all...

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takes place after dark.

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The Northern Lights.

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One of the planet's
greatest visual displays.

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But it's recently been discovered

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that the Earth's ultimate light show
produces secretive sound signals too.

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These eerie whistles
are usually inaudible to the human ear,

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but can be picked up
by long wave radio receivers.

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Occurring up to 400 miles overhead,

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the signals are produced

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as charged particles from the sun
collide with our atmosphere.

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The mesmerizing song of the polar night.

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From celestial symphonies,

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to the most heartwarming
of all Arctic sounds.

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Each spring, love is in the air

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as thousands of eider ducks
descend on the sea ice

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to breed.

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Males sport unmistakable
black-and-white plumage

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with a green flash,

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while females
are a more understated brown.

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With so many eager rivals,

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males go all out to impress

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with their rather charming mating calls.

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Creating the sound requires a strong,
highly flexible neck.

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In fact,

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male neck muscles are 50% thicker
than those of females.

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The call may sound unusual to us,

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but for highly competitive males...

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the noise triggers a chorus of rivals.

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Should this fail
to get the female's attention,

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the Romeos have another seductive trick
to play.

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Flapping wings is one of a dozen
eider duck courting rituals.

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Duck dad dancing.

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If a female accepts,

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she does so with a simple snort.

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The quirky calls
have paired another couple.

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It's a date.

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Just one of the surprising ways
animals use sub-zero sound

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to thrive at the Earth's frozen poles.

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To capture the sounds of the polar regions
in more detail than ever before,

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the <i>Earthsounds</i> team underwent
the 10,000-mile journey to Antarctica.

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They were to spend a month at a research
base called Neumeyer Station III.

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Day one.

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The team strikes out in search
of emperor penguins.

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When you first arrive
at the penguin colony,

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the first thing that hits you
is the sound of it.

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{\an8}Thousands of birds all calling
at the same time.

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Amidst this cacophony...

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the challenge is to get a clean recording
of a hungry chick's signature whistle.

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There's thousands of chicks
and adults, all calling together.

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So, the difficulty is going to be
picking out an individual call.

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But on a continent
where humans are something of a rarity,

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the subjects turn out to be
most accommodating.

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We're meant to be staying 30 meters
from the colony,

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and five meters from any penguins.

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But this inquisitive chick
has come forward.

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So, it might be a bit easier to get
the sound than we thought.

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Well, that was straightforward.

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The next sound the team
wants to record is underwater.

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And their luck continues,
with another obliging performer.

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That's a Weddell seal song.

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Moments later,
the source of the incredible calls...

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makes an appearance.

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That's a mom and pup.

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Clearly we've come
to occupy their breathing hole,

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and they just wanted to check us out.

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It's so incredible that in Antarctica,
the wildlife comes straight to you.

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But the team's run of good luck
is about to end abruptly...

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as conditions take a turn.

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{\an8}The weather has come in pretty quickly.

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{\an8}There's a whole bunch of snow around us,
and the visibility has got a lot worse.

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So, time to go back to base.

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00:26:38,599 --> 00:26:41,518
Antarctica is the windiest place on earth.

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And in recent years,

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our warming climate
has made the winds even fiercer.

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00:26:52,654 --> 00:26:57,451
Gales here have been known
to reach 200 miles an hour.

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The crew hunker down.

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00:27:06,877 --> 00:27:10,714
Three days later,
the weather finally clears.

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And the team gets recording again.

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Starting to build a library
of Antarctica's more unexpected noises.

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I've really enjoyed
some of the smaller sounds.

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The sound of penguins' feet on the ice
as their claws crunch through the snow.

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00:27:49,795 --> 00:27:52,464
The incredible underwater hubbub
of air bubbles

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as the birds streak up to the surface.

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00:27:59,096 --> 00:28:03,267
Then there's a sound like a squeaky toy
as they jump out of the water.

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It sounds like the air
is leaving their body.

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00:28:11,984 --> 00:28:14,236
After four weeks on the ice,

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the team have captured
the Antarctic soundscape

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closer than ever before.

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00:28:26,415 --> 00:28:30,252
A unique insight into the magical world

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of sub-zero sound.