The Himalayas.
The highest mountain range on Earth.
With peaks rising over five miles into the clouds.
Here, deep canyons create the world's largest echo chamber,
where the slightest noise ricochets through the valleys.
For the few extraordinary animals that live here,
sound is key to their survival.
A snow leopard.
The ghost of the mountains,
and the Himalayas' ultimate stealth hunter.
This male is searching for a meal.
To get close to his prey,
it's vital that he avoids being seen.
But in the silence of the mountains,
it's just as important that he's not heard either.
With prey scattered far and wide,
snow leopards take advantage of the unique high-altitude soundscape.
Their hearing is three times more sensitive than ours.
Allowing them to pick out faint noises from miles away.
And they listen out for one particular animal.
Himalayan ibex.
As the hunter closes in, he switches to stealth mode.
An ambush predator, he needs to get within just a few feet.
The long fur between the pads of his toes
helps soften his footsteps, allowing him to sneak closer.
All it takes is one loose rock...
and the ibex will be spooked.
Cover blown.
He's forced to attack from a hundred feet.
In the thin mountain air, every stride uses precious energy.
On the sheer cliffs, all it takes is one slip...
and the chase is over.
This time, he goes hungry.
But in just a few weeks, he'll use the mountains echo chamber
for a very different type of pursuit.
Beyond the peaks lies a whole new soundscape,
where the acoustic rules are turned upside down.
The Tibetan plateau.
On these flat, featureless planes,
sound doesn't echo around but dissipates into the cold, dry air.
So up here, you've got to shout to make yourself heard.
The chiru, perhaps the world's strangest antelope.
Each winter, herds of these curious creatures gather to breed.
This male guards a harem of ten females.
And to keep them close...
he struts his stuff.
He needs to watch out
as he's not the only male with mating on his mind.
Rivals are gathering and fighting amongst themselves.
The winner will get to challenge our male for his harem.
So he sends them a blunt reminder of who's boss.
Deep sounds travel further.
So chiru use low, loud growls to broadcast their warnings for nearly a mile.
But not every male is intimidated.
A rival roars in reply.
It's a direct challenge our male will have to silence.
And so begins the battle of the bellows.
Possibly the world's worst singing contest.
On most occasions, these growl-offs are enough to settle any dispute.
But today our male has met his vocal match.
Now, with his harem at stake, he must see the intruder off.
Even with little oxygen in the air, chiru can run at 50 miles an hour.
This time, his rival is all talk.
In the Himalayas, conditions can change in an instant...
shifting the soundscape.
A fresh fall of snow muffles sound,
making the mountains quieter.
Now, to hear any signs of life, you must listen even more carefully.
Beneath the surface, microphones pick up the secretive chatter
of one of the Himalayas' cutest-looking creatures.
The plateau pika.
These social little mammals live in family groups around 20 strong.
They use a repertoire of seven different calls to keep in touch...
and to settle disputes with neighbors.
But all this chatter attracts unwanted attention.
A Tibetan fox.
Foxes have excellent hearing, six times more sensitive than our own.
They're tuned in to high-frequency sounds like the squeaking of pika.
Honing in, the fox starts to stalk.
Keeping a low profile and moving slowly
to minimize making any noise.
Their ears can rotate independently,
helping them pinpoint the slightest sound...
and creep in close.
This unsuspecting pika is just inches from the fox's jaws.
But luckily, pika have a neighborhood watch scheme like no other.
Their squeaky alarm calls are highly directional,
helping them pinpoint where the warning is coming from...
so they can make their escape.
But the wily fox doesn't give up easily.
Once back in their burrows, the pika start chatting again.
And the hunter listens in from above.
He starts to dig.
He can burrow three feet into the ground.
Now, it seems the pika are in serious trouble.
But these smart little mammals are one step ahead.
Because their tunnels have emergency exits
so they can escape even the most determined fox.
The pika live to squeak another day.
In the Himalayas, the loudest sounds of all
come from the peaks themselves.
Weather fronts collide with mountains, weakening rock faces.
Glaciers groan as they carve deep valleys.
And avalanches thunder down slopes.
Every year, these peaks get half an inch taller.
The sounds of the world's highest mountains growing even higher.
But in the foothills, one noise can be heard above all others.
Fueled by meltwater and monsoon rains,
fast-flowing mountain rivers charge with white water...
creating a background din as loud as a jet engine.
This drowns out any other sounds and makes communication a challenge.
But one little bird has found a way to cut through the chaos.
A Himalayan dipper.
This female is hunting for aquatic insects...
to feed her hungry chick.
Even though he's just a few days old,
he's already a whopper with an appetite to match.
When close by, they easily call to each other.
But to hunt for food, Mum must travel far along the river.
And that makes keeping in touch much harder.
Especially if your chick is a little clumsy.
He's swallowed up by the torrent and washed downstream.
When Mum returns, her chick is nowhere to be seen.
So she produces a high-pitched call.
By blocking out the background noise... we can tune into it.
The dipper's vocalizations are way above the range of the water.
A bandwidth that scientists term "the silent window."
It's their own communication channel that cuts above the background din
so her chick can still hear her.
Even at such a young age, he's already found his voice and calls back.
High-pitched sounds are easier to locate.
So, from his call alone, Mum can track him down.
And the pair are reunited.
But for Mum, there's no time to rest.
Of all the sounds in the Himalayas,
there's one special call that's rarer than any other.
The love song of the ghost of the mountains.
For two years, a remote surveillance network
eavesdropped on the highest peaks.
And recorded rare footage of elusive snow leopards.
Each spring, for a few weeks, females begin an annual search for a mate.
A seemingly impossible challenge given the vastness of the mountains.
The first step is to leave a scent trail.
Wandering huge distances, they mark across their territories
of over a hundred square miles.
And this is what they're hoping for.
A young male.
As he rolls in the scent,
we can hear his intimate call, known as a chuffle.
Having drawn a male close...
the female now switches from scent to sound.
Her song can travel for miles.
For a male, it's an irresistible draw.
And just inches from our camera,
his reply is the closest ever recording of a snow leopard's call.
Their haunting duet lasts for just a few minutes.
But it's enough to bring two solitary cats together.
Having spent most of their lives alone...
this new couple appears a little nervous.
Their tender courtship is a rarely seen moment.
Thanks to the mountain's unique soundscape,
snow leopards are able to find love...
in the vastness of the Himalayas.
To capture the mountain's rarest call,
the Earthsounds team traveled to Ladakh in northern India.
At over 15,000 feet.
Where's the chairlift? They said there'd be a chairlift.
They hope to record the sound
of one of the world's most elusive animals,
the snow leopard.
These secretive big cats only call during the few weeks of their breeding season.
So being in the right place at the right time would be a huge challenge.
They'd need the help of expert guide, Khenrab Phuntsog.
The landscape is so vast.
You'll have to be exactly in a place where they make the call.
Which is very difficult to predict.
Their first task was to find the cats.
Not an easy job in the world's largest mountain range.
Yeah.
But for Khenrab, these peaks are full of hidden signs
that snow leopards leave behind.
So it went down this way.
- It's very clear, isn't it, in snow? - Yeah.
Good sign that cats are around.
But finding the snow leopards is just the start.
To capture their vocalizations, the team has to rely on remote camera traps.
Here we're burying it in the ground with the hope that we'll get the--
one of the snow leopards walking over the top of it.
And I guess I would just stop about here.
Yeah.
And then I'd walk over the top.
- And then there's that tail's trail-- - Yeah.
That'd be cool.
And to guarantee they get the sound they've come for,
Nick has a plan.
Just looking for somewhere to place this little remote sound recorder.
This is like a camera trap, but it's just for the sound.
Dot these around and bug the mountain.
Once everything is set and tested...
It's good. It's okay.
...all the team can do is head back down the mountain and wait.
After three weeks, it's time to check whether they've struck lucky.
Something is coming.
And the team get their first glimpse of a snow leopard.
But despite getting closer to these mysterious cats than they ever imagined...
Nick's sound recordings were silent.
I've been doing this too long to get too disappointed.
You take the rough with the smooth when you're camera trapping.
But it is disappointing.
Capturing the rarest call in the Himalayas was never going to be easy.
Maintaining all the traps would require the equivalent effort
of the crew climbing Everest four times.
Finally, two years later, Khenrab hits the jackpot.
A male snow leopard triggers the camera.
And as he leaves the shot, he begins to call.
The clearest and closest ever recording of a wild snow leopard call.
The extraordinary love song of the ghost of the mountains.