Planet Earth III

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.

The rainforest of Costa Rica.

As you might expect, it rains most days.

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.

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.

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.

It's the middle of the dry season.

This is the only remaining water hole for miles around.

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.

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.

The Okavango Delta.

These waters attract millions of animals.

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

A lily-trotter.

And he is a father...

..on duty.

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.

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.

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.

And it's risky.

Get close enough to distract...

..and hopefully divert the crocodile.

Mission accomplished.

He signals the all clear.

But one chick is missing.

His calls...

..go unanswered.

A little bedraggled but alive and well.

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...

..these residents of the delta are not.

African painted dogs.

Normally, they are remarkably successful hunters.

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

Finding prey isn't the problem.

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

But between them is deep water.

It's not safe to cross here.

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.

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.