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.
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.
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.
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.
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...
..but at a cost to the natural world.
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.
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.
Out of water, she cannot survive for long.
The journey back to the river will take hours.
Her life now depends on these people.
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.
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.
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.
Have they got the dolphin? Yeah, I think so. They've got it.
Now speed is key.
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.
But rescue missions don't always go to plan.
A puncture.
The dolphin is quickly moved to the film crew's truck.
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.
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...
..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.