All language subtitles for Planet.Earth.III.S01E04.2160p.iP.WEB-DL.DDP.5.1.HLG.HEVC.HuN-TRiNiTY_Track4

Beneath the Yucatan Peninsula
of Mexico...

..lies another world.

A labyrinth of caves...

..carved by freshwater...

..and essential for life on Earth.

Freshwater shapes the land
and the creatures that live in it...

..in the most surprising ways.

CHIRPING

The rainforest of Costa Rica.

As you might expect,
it rains most days.

RUMBLE OF THUNDER

But it's only during the wet season

that the rains become torrential.

And with them...

..comes a most remarkable creature.

Gliding tree frogs.

Thousands drop in...

..to gather at this
newly-formed pond.

They all have an urgent need.

To breed.

This female needs to find a leaf

and lay her eggs.

FROGS CROAK

The breeding frenzy
will only last one day...

..so she needs to get a move on.

There are lots of eager males.

Nine for every female.

And all are ready to
spring into action.

So, for her, getting
a partner is no problem.

But the other males are desperate.

For this female,
mating is rather chaotic.

Things are getting out of hand.

There is one way to deal with this.

A few well-placed kicks...

..usually does the job.

Problem solved.

FROGS CROAK

She and her partner can now
finally mate...

..in relative peace.

The eggs are positioned perfectly...

..directly overhanging the pond.

This is the ideal nursery
for tadpoles.

For now, her work here is done.

She'll be back next year
when the heavy rains return.

As quickly as freshwater appears...

..it can vanish.

Southeast Sri Lanka.

FLIES BUZZ

It's the middle of the dry season.

WIND HOWLS

This is the only remaining
water hole for miles around.

DEER YELPS

Mugger crocodiles -

giants, five metres long.

Dozens are hiding...

..beneath the surface.

And others...

..are on their way.

This male has been
travelling for hours.

The water hole is the only place
where he has a chance

to catch a deer.

A single adult would be enough to
feed him for the entire dry season.

But hunting prey that
knows you're here...

..requires something
truly remarkable.

He creates...

..a trap.

He digs down into the mud...

..deep enough to hide
beneath the vegetation.

Perfect camouflage.

He has built his trap
right on the water's edge...

..so it's the first place that
thirsty deer will come to drink.

SQUAWKING

These crocodiles
have learned to exploit

the deer's desperate need
for freshwater.

Freshwater transforms not only
the lives of animals...

..but entire landscapes.

Every year,
over a trillion litres of water

flow into the Kalahari Desert.

It has travelled a thousand miles
from where it fell as rain.

And on arrival...

..it turns the Kalahari...

..into a vast oasis.

SQUAWKING

The Okavango Delta.

These waters attract
millions of animals.

And for some, now is
the perfect time to raise a family.

A lily-trotter.

CRACKING

And he is a father...
CHIRPING

..on duty.

SQUAWKING

He has not just one...

..but four newly-hatched chicks.

They won't be able to fly
for another six weeks...

..so, until then, he alone
is their protector.

CHIRPING

Walking on water
is not straightforward...

..so he must show them how
to be a lily-trotter.

The floodwaters make the perfect
training ground.

And in two weeks...

..his chicks not only
double in size...

..they grow in confidence.

But this is a dangerous place
to stray.

SQUAWKING

He sounds the alarm,

and his chicks instantly freeze.

They can't yet fly to safety.

But there is one thing
he can do to protect them.

SQUAWKING

And it's risky.

SQUAWKING

Get close enough to distract...

..and hopefully divert
the crocodile.

SQUAWKING

SQUAWKING

Mission accomplished.

SQUAWKING

He signals the all clear.

But one chick is missing.

SQUAWKING

His calls...
SQUAWKING

..go unanswered.

A little bedraggled
but alive and well.

CHIRPING

His chick just hadn't heard
his call.

And when danger threatens,

its always better
to be safe than sorry.

He's brought his chicks,
all four of them,

to the verge of independence.

Whilst lily-trotters
are well adapted

to life in this flooded world...

PANTING

..these residents of the delta
are not.

HIGH-PITCHED BARKING

African painted dogs.

Normally, they are remarkably
successful hunters.

But this pack of five
hasn't eaten in days.

PANTING

Finding prey isn't the problem.

This herd of lechwe is only
a few hundred metres away.

But between them is deep water.

HIGH-PITCHED WHINING

GRUNTING

HIGH-PITCHED SQUEALS

It's not safe to cross here.

LECHWE BARKS

On dry land,
the dogs can run at over 40mph.

But when threatened, lechwe always
seek the safety of water.

With their splayed hooves
and long hind legs,

they can power their way
through the water.

The dogs can't.

The pack won't be able
to outswim the lechwe.

PANTING

But they do have a strategy.

The dogs drive the lechwe to
where they have the advantage.

Back towards dry land.

Now the pack can regain lost ground.

Success today, but tomorrow
the pack will need to hunt again.

In the Okavango,

animals must always adapt to the
coming and going of freshwater.

But there are a few rare places,
as here in Lake Malawi,

where freshwater has lain
for millions of years.

And that has enabled life to evolve

into a dazzling diversity
of species.

There are over a thousand
different kinds of fish here...

..more than any other lake on Earth.

Competition in these waters
is intense.

Meet Nimbochromis livingstonii.

A master of deception

that tricks other fish
into thinking it's dead.

Its mottled colouration
mimics decaying flesh.

But to appear dead...

..one must at least...

..act dead.

This juvenile is fooling no-one.

Perhaps faking his death
will be more believable.

Blown it.

A more subtle performance
will be needed.

This is more convincing.

But his aim must be better
than that.

Nearly.

It seems he's overstayed
his welcome.

In a final effort...

..he appears to even
hold his breath.

Success at last.

All it took was a million years
of evolution...

..and the unique stability
of this ancient lake.

Off the western coast
of Equatorial Africa

lies the volcanic island of Bioko.

RUMBLE OF THUNDER

Freshwater here has
a turbulent nature.

Few venture into such waters,

but for those that do,
there can be great rewards.

After months feeding and growing
out at sea,

these gobies are ready to breed.

And the safest place
to lay their eggs

is somewhere that
ocean predators can't reach.

The top of this waterfall.

Surely an impossible journey
for a fish.

But over the coming days,
the gobies transform themselves.

They change colour.

Their mouth rotates and acts
like a kind of suction cup...

..which they will certainly need.

Because the only way...is up.

It is 30 metres to the top.

They are only an inch long.

This is the equivalent of
a human being scaling a waterfall

over a mile high.

Millions attempt the climb...

..but less than 1% will make it.

This goby has been
climbing for days.

Made it!

Those that reach the top
can now prepare to breed.

And when the time comes, their young
will be washed back out to sea.

The extraordinary power
of flowing water

makes it our planet's
greatest architect.

This may look like the surface
of an alien planet...

..but these strange patterns
are channels of freshwater...

.flowing towards the sea.

For 3.8 billion years,

ever since the first rains fell...

..freshwater has been
making this journey.

But today, two thirds
of our world's great rivers

no longer reach the sea.

CREAKING

There is a finite amount
of freshwater on Earth...

..so humans have devised
extraordinary means

to ensure that it is always
available to meet our every demand.

We now control freshwater...

..on a colossal scale.

The largest irrigation system
on Earth

has been built here in Pakistan.

HORNS BLARE

A vast network of dams and canals...

..that diverts and drains
the great Indus River.

It's used to irrigate an area
the size of England.

This man-made freshwater world

provides water and a livelihood

for millions of people...

LAUGHTER

CHATTER

..but at a cost
to the natural world.

CHATTER

A rare creature has become stranded
in this irrigation canal.

And the water will soon be gone,

used to supply the surrounding land.

Endangered, they number fewer
than 2,000 individuals.

An Indus river dolphin.

CHATTER, YELLING

This female dolphin is 150 miles
from the main river.

She can't return on her own,

so the only option is for
this rescue team to take her.

CHATTER

CHATTER CONTINUES

Out of water,
she cannot survive for long.

CHATTER

The journey back to the river
will take hours.

CHATTER

Her life now depends
on these people.

SIREN WAILS

CHATTER

Nearly 30 dolphins had to be
rescued in one month alone.

Freshwater is not only
a resource for humans.

It is the home for
countless species

that simply cannot survive
without it.

Freshwater is the lifeblood

of Planet Earth.

To film the story of the most
endangered freshwater dolphin,

the Planet Earth team joined the
world's leading expert - Uzma Khan.

The first time I saw Indus dolphin,
I was just amazed.

It's a beautiful creature, you know,

it's like love at first sight.
It's so unique.

Uzma has been studying
the last remaining population

for over 20 years.

There's a dolphin that surfaced
just right there.

Indus river dolphin
can only live in freshwater.

It is only found in Pakistan
and nowhere else.

And we only have about 2,000
of them left in the Indus River.

So, you know, each individual
is special.

Following the dolphins in the murky
water is extremely challenging.

They only surface for a split second

and that makes protecting them
difficult, too.

Their greatest threat is becoming
trapped in the irrigation canals.

But Uzma may have a solution,

and teams up with a rescue unit...

..to attempt something
never done before.

This is the first time
we'll be putting satellite tags

on river dolphins in Asia.

Sensors has not been tested
on this species,

there's a little bit of anxiety,
but if they work,

then it's going to be a breakthrough
for conservation.

SIREN WAILS

Uzma has been developing
a suitable tag for over ten years

and hopes it will help her
collect critical data

on how the dolphins are moving
through the dams and canals.

But in order to tag, she will
first need a successful rescue.

CHATTER

I'm usually very concerned
when a rescue is happening.

You know, the divers,
they are very experienced,

but a part of me is always
very worried.

They are wild animals
that have never been handled.

It's really quite a risky thing
that they're undertaking here.

There's a good chance
the dolphin might die.

But, of course, they're balancing
that against the fact

that they definitely will die
if they stay in here,

so it's a risk they have to take.

CHATTER

Have they got the dolphin?
Yeah, I think so. They've got it.

Now speed is key.

CHATTER

We're now bombing across the field
on the way to the release site,

and I think Uzma went ahead of us,
so I'm really hopeful

that she'll get her opportunity
to tag the dolphin.

SIREN WAILS

But rescue missions
don't always go to plan.

LOUD RELEASE OF AIR

A puncture.

The dolphin is quickly moved
to the film crew's truck.

ANXIOUS CHATTER

But with no roof for shelter,

the dolphin is at greater risk
of dehydration.

The stakes are high, even right now.

It's a cute little animal
and we don't want it to die.

The team re-route
to the closest release site.

There can be no more delay.

Uzma calls off the tagging.

It's unfortunate
because we were all prepared,

we were very excited since morning,
all geared up,

but hopefully we'll get an
opportunity to tag the next rescue.

A few days later,
Uzma finally gets her chance.

It's a very exciting day for us.

It's OK.

The moment that we have been
waiting for, over ten years.

Just stay calm.

OK, it's good to go.

Congratulations.

Well done.

I am very happy, very happy.

I think it's a great accomplishment.

It's the happiest moment of my life.

APPLAUSE

Within a few weeks, the tags begin
to reveal some new insights.

They are moving quite a lot,
which we never expected.

So one of the dolphins actually
went about 46km upstream

from the release site, and the
other thing is that we thought

that they will stay
in the main river stream,

but they are using side channels.
It is extremely interesting.

Tracking the movement
of the tagged dolphins

will help Uzma better understand

when and where
they become trapped...

..helping safeguard the future
of this most endangered animal.

Next time...

..forests.

A hidden world...

CHIRPING

..where lives are
intimately entwined...

..in the most unexpected ways.

Habitat Explorer brings animals
and their habitats to life.

Explore this free interactive
and make origami animals.

Go to bbc.co.uk/planetearth3

and follow the links
to the Open University.

Or to order a free printed version,

visit the website or call
the number on the screen.