Original subtitles

Of all our planet's forces,

perhaps none has greater power over us than water.

For me, water's the most magical force on Earth.

The presence of water shapes, renews and nourishes our planet.

Oh, my gosh! You're getting all wet there!

It's our planet's lifeblood.

It pumps through it continuously,

delivering vital ingredients for life.

Ah, it's glorious.

Water makes Earth alive.

Yet water is just one of the ways

that the power of the planet has shaped our lives.

The Earth has immense power...

...and yet that's rarely mentioned in our history books.

I'm here to change that.

In this series, I'm exploring four great planetary forces

that have influenced our history.

The power of the deep Earth...

...that fuelled technological innovation.

Wind.

It has shaped the fate of entire continents.

And fire...

...which gave us the power to conquer the planet.

But I'm going to start with water.

The magic of water is that it's constantly transforming itself,

shifting between guises and from place to place.

Our struggle to control it has been behind the rise and fall

of some of the greatest civilisations on Earth.

The centre of the Sahara Desert in North Africa.

One of the driest places on Earth.

I'm over six hours'drive from civilisation.

Temperatures here regularly reach 4o degrees Celsius,

and there's less than a centimetre of rainfall each year.

Ah....

The whole thing's moving.

(HE STRAINS)

It's like walking on water.

Yet hidden amongst these dry dunes are clues

that point to the dramatic influence the planet has had on human lives.

I've come here because although you'd never know it,

the story of this place is all about water.

The clues are etched into that rock face there.

Prehistoric rock art dating back 6,OOO years,

and depicting the most unlikely cast of characters you've ever seen.

Wow, what is that?

It's a giraffe...

It's a giraffe, look at it, there's the neck.

There's its ears, that's an eye,

and its mouth.

That's really natural, isn't it?

And this looks like the giraffe dipping its head down,

drinking some water - we've got a herd of giraffes here!

There's two cats.

They're fighting.

This... What is this?

It looks like the figure of a man, but he's wearing a bikini.

And this is clearly a crocodile, which is especially odd here.

This is an aquatic animal, it doesn't just paddle around.

It needs a lot of water to live in. In fact,

all the creatures that are depicted on these rocks are not desert animals -

they need wet conditions.

In such a parched wilderness, how can this be?

The only explanation is that 6,ooo years ago, this place was wet.

Once you know what to look for, the evidence is all around.

Up there is a river valley that's been carved out into the rock,

and it's been carved by running water

which has flowed down here, smoothing off this rock bed,

and then cascaded down into the valley and off there.

6,OOO years ago, that was a big river.

Satellite images reveal that the river bed I'm standing in

is just one of a network of past river valleys

that crisscross the Sahara Desert.

1 o,ooo years ago, this dry, empty place was entirely different.

Little is known about the early Saharans who lived here then,

but we do know that they depended entirely on water.

Water formed the lakes in which they swam.

Water nourished the plants which fed the animals they hunted.

Water filled the clay pots from which they drank.

But then the climate changed.

About 5,5OO years ago, the Sahara began to dry.

The rains failed, the rivers shrank, and the lakes dried out.

For the early Saharan people there was only one option -

to follow the rains and abandon the desert.

The fortunes of the early Saharan people

reveal a universal, timeless truth -

our fate is inextricably linked to water.

The problem is, water never stands still.

It's always on the move across the planet.

We think of this as a blue planet.

But while water is abundant, most of it is no use.

More than 97% of the Earth's water is salty ocean, which we can't drink

or use to grow crops.

Less than 3% is fresh water, on which all human life hangs.

What's more, that tiny fraction is often hard to pin down,

because fresh water has a life cycle all of its own.

I'm about to explore that cycle, in all its elusive glory.

You know, water seems so familiar, doesn't it?

But to see its remarkable qualities

you have to go to some extreme lengths.

(MOTOR CHUGS INTO LIFE)

(REWING)

Here we go...

Ho-ho! Feel that!

( # WAGNER: Ride Of The Valkyries)

Here we go!

Oh... Hey-hey! Oh, we're off!

Oh, my God!

It's a bit bouncy!

I shouldn't have had that bacon and eggs this morning.

O-o-o-h! (LAUGHS)

The fresh water that we depend on begins its life in the oceans.

As the sun's rays beat down on the surface of the sea,

they heat the water molecules until some evaporate.

It's the start of an extraordinary journey.

You know, when water evaporates,

it feels as if it vanishes into thin air.

But although we barely notice it,

water molecules are suspended around us all the time.

It's just that we're only aware of it when they clump together

as cloud.

At any one time, less than a thousandth of the world's fresh water is up here

in the atmosphere.

It may not seem much, but this is what spreads water from the seas to the land.

A water molecule doesn't hang around up here for very long.

In fact, it spends less time up here in the atmosphere

than at any other time on its journey -

a mere nine days

until the typical water molecule crashes to Earth as rain.

(THUNDER RUMBLES)

(BIRD SQUAWKS)

For most of us, rain is perhaps

the most familiar stage of the water cycle,

but notoriously the least reliable.

As the water falls as rain, it joins a bigger system,

cascading and carving its way across the land surface as streams and rivers.

Look at that!

Water absolutely everywhere!

Rivers and rain are the part of the water cycle that we depend on.

Whoo-hoo!

And yet they're only a tiny proportion of the world's fresh water...

...a measly 2% of all fresh water on the planet.

The rest of the Earth's fresh water

is locked away down there, on the ground.

Oh... (LAUGHING)

Oh!

What a landing!

The vast majority of it is stored as ice.

Most of the rest seeps deep into the Earth,

where it's known as groundwater.

Hidden away down here

is the planet's second-largest store of fresh water.

But in the end, all water arrives back in the oceans,

and the cycle begins again.

What that circulation means for us humans

is that water is a moving target.

We constantly have to seek it out on its endless cycle and intercept it

wherever and whenever we can.

This quest to...

to pin down water has played a defining role in human history.

You can trace the impact of our quest for water

right back to the dawn of civilisation,

about 1 2,ooo years ago.

It all began with a big block of ice.

1 2,ooo years ago,

much of the northern hemisphere was covered in a single, huge ice sheet.

And even today you can see its legacy...

...here in Iceland.

This glacier is a tiny remnant of that once enormous expanse of ice.

Ice is like a storage cupboard in the circulation of water around the planet,

a store into which water can be deposited or withdrawn.

And it was a shift in the amount of water locked up here

that was to drive one of the greatest ever transformations of human society.

Today, the ice sheet here is melting and retreating,

and releasing this great armada of icebergs.

But if you go back 1 2,5OO years ago,

it's a very different story.

Then the ice was expanding,

sucking moisture out of the atmosphere in vast quantities

and locking it away in the ice.

And the effects of that were felt right across the planet.

Thousands of kilometres away in the Middle East...

...it led to a drought which lasted for centuries.

It had its most profound impact in what would become known

as the Fertile Crescent, an area famed for its exceptionally rich soil.

This drought would trigger the start of the defining characteristic

of human civilisation.

Back then, every human on the planet was a hunter-gatherer.

Those living in the Fertile Crescent, the Natufians, thrived on rich pickings

of fruit and berries, with plenty of deer and ibex to hunt.

But as the drought took hold, to survive they would have to adapt.

They came up with two distinct strategies.

One group developed this, the Harif point,

a new, state-of-the-art arrowhead

that allowed them to tackle a drought by hunting more efficiently.

But a second group came up with something a little bit more subtle.

Although you wouldn't know it, this is a sickle,

and it offered a completely new approach to gathering food.

This small, stone blade represented

a decision not to chase food, but to stay put and grow it.

The Harif point did a good job for the hunters.

But it was the sickle that really changed history.

In a drought, it's safer to stay close to water,

but that decision to remain in one place meant planting crops was essential.

If you go foraging in the forest,

you can only collect so much food with your bare hands,

but if you've got one of these, you can harvest fast and furious,

and for the same amount of effort, you can collect far more food.

With this simple tool,

these people had begun the agricultural revolution.

And the rest, as they say, is history.

A lack of water and a simple but ingenious response

led to the birth of civilisation.

But once farming took hold, it had a profound impact

on our relationship with water.

No longer could we simply follow the rains.

Now people needed regular, reliable sources of water

to make sure their crops grew.

So the need for water began to define

where the first civilisations could flourish.

That led people to the one stage of the water cycle

that offers reliable fresh water -

rivers.

Across the planet,

rivers cover a tiny proportion of the Earth's surface,

but for the first farmers, they became magnets.

But rivers did more than supply a steady source of water.

They changed the very character of the civilisations

that grew up along them,

influencing everything from politics to social organisation.

The power of rivers to shape history is graphically illustrated

by perhaps the greatest of all early civilisations...

...Ancient Egypt.

You might think you know the story -

a mighty civilisation that built the pyramids

under the autocratic rule of ruthless Pharaohs.

But if you want to understand what really made Egypt great,

you have to leave the pyramids and the temples behind...

...and come here, to a small place that hardly anyone visits.

You know, at first glance these look like your average, everyday,

2,OOO-year-old steps.

But this staircase is what made Ancient Egypt tick.

You get an idea of its true purpose by the markings on the side wall -

these grooves were carefully carved into the marble -

because this was a beautifully simple measuring device.

And to see what it was measuring, you have to pop round the corner.

Oh!

It's all wet!

And this is it -

the Nile river.

That set of steps and markings is a Nilometer.

It measured the changing level of the river.

Each year when it flooded, the maximum height that the waters came to

would directly predict the yield of the crops

and, with that, the profits that the farmers made.

It worked because the water of the river

carried something special within it -

an almost invisible treasure

that was the secret of Egypt's economic might.

What made Egypt great is this stuff -

silt.

It's a rich soup of minerals, which...

It's like an espresso.

Tiny flecks of rock and minerals that the river picked up

over its wandering course and swept along with the flow.

All rivers carry some silt,

but the Nile has the benefit of starting in Ethiopia,

where the rock is young and volcanic.

This forms the richest of silts.

1 4o million tonnes of the stuff are carried by the Nile down river

to Egypt each year.

Every year, the seasonal flood covered the fields

and left behind nutrient-rich silt that fertilised the crops.

The more silt, the more food was produced.

It was the size of the flood -

and with it the bounty of silt - that the Nilometer was used to predict.

So, simply by measuring the height of the Nile, the Egyptians were able

to forecast food production and, with it, the profits of the farmers.

Each year, they used this information to set tax levels.

So the wealth and the might and the splendour of Ancient Egypt

is all down to a simple twist of geographical fate.

In fact, Ethiopia itself gets almost no benefit

from that fertile soil washed from its highlands.

It's even said that its greatest export is the silt

that it sends down the Nile, silt that made the Pharaohs rich.

But the ebb and flow of the Nile

had more far-reaching implications for the Egyptian people than mere taxes.

Intriguingly, it may be that where access to water is limited,

that actually determines

the way a society is organised and even its use of slavery.

Where water is in short supply -

or from a single source, as it is in Egypt -

then you need a highly structured society to get the best out of it.

For large-scale irrigation, you need bureaucrats to decide

where to dig the water channels.

You need teams of working men - slaves, really -

to do the actual hard work of digging.

And once the channels are in place, you need farmers with money enough

to buy the water it's delivered.

So right away you've got three tiers of society,

and I haven't even mentioned the Pharaohs.

So the rigid, hierarchical structure of Egyptian society

wasn't just dictated by the Pharaohs.

It also emerged because the Egyptians had only one water source -

the Nile.

5,ooo years ago,

it wasn't just the Ancient Egyptians who noticed the value of rivers.

Other great civilisations were also forming along the banks of rivers.

In Mesopotamia,

the Sumerian civilisation flourished between the Tigris and the Euphrates.

Further east, the Harappan civilisation formed by the Indus.

And early Chinese civilisations were emerging along the Yellow River.

But not all early farmers were content to settle by rivers.

Others learned to exploit new sources of water,

in the unlikeliest places.

Like the Sahara Desert, in Libya.

These are the remains of the ancient city of Garama,

which about 2,5oo years ago was the centre of a powerful empire.

Today, it's a bit of a maze, but from up here

you can see the shapes of the buildings, the way the streets interconnect.

You get a real sense of how this place must have worked in its prime.

This was the home of the Garamantians...

...which, for me, are a rather forgotten people.

They've been eclipsed in the history books by their showy contemporaries,

the Greeks and the Romans.

The Garamantians dominated the Sahara Desert for almost 2,OOO years.

They were the society that first brought civilisation to the desert.

Far from just scraping by in this harsh landscape,

the Garamantes were flourishing.

They grew crops such as cereals and grapes.

They kept horses and pigs.

Clearly, they needed large amounts of water.

So where did they find it, here in the middle of the desert?

Now, this is the key to the Garamantians' incredible success.

It's vertical holes that are sunk deep into the ground...

...4O to 5O metres - that's about 1 5O feet.

And the purpose of them was pretty simple -

it was to bring water up from below ground.

In this environment, it must have seemed like it was almost magic.

In fact, the Garamantians had discovered groundwater.

Beneath the surface of the Sahara is a surprising part

of the great water cycle -

a massive store of groundwater.

This is water that has seeped into the ground

and has collected in porous layers of rock.

The water came from the period

thousands of years before, when the Sahara was lush and wet.

Some of that water percolated into the rocks below and remained there,

despite the dramatic drying above...

...until the Garamantes found it.

You kind of dig them down until you hit the water table

and then you just keep doing the same thing.

There's one after another, after another, all in a whole line.

But these holes aren't wells - they're maintenance shafts.

They reach down to tunnels which carried the water.

The point is, right up there at the end is where the water source is,

so the water flows naturally from the escarpment up there, underground,

down to the kind of oasis over there.

Now, that's where the Garamantians' city was.

What they could have done is they could have dug wells down

and lifted the water out, but that's a lot of work for very little return.

Much better to use gravity to channel the water

in an underground tunnel straight to where they need it.

That was the Garamantians' real ingenuity.

The Garamantes had managed to tap the same water

that the early Saharans had enjoyed thousands of years earlier.

By mining groundwater, the Garamantians managed

to turn the clock back on the Sahara - they made the desert bloom again.

But the human struggle to pin down water

is forever balanced on a knife edge.

Get that balance wrong and you pay the price.

For all their ingenuity, the Garamantes over-exploited their groundwater.

Eventually it ran out, and so did their civilisation.

Now all that remains are the bats.

(SQUEAKING AND FLUTTERING)

Today, modern Libyans have tapped into this same groundwater supply,

by using pumps to reach deeper than the Garamantes could.

But just like their ancient predecessors, they're exploiting a finite resource.

At most, it will last only another 5o years.

But water in this most inaccessible stage of the water cycle

is found in many other places.

It's at its most spectacular in Tallahassee, in Florida.

Here, divers are just beginning to explore a mysterious series of caves

called a karst system, carved out by groundwater over millions of years.

This is one of the planet's least known frontiers.

When they began, these divers had no idea

of the extent of the cave network.

To explore these caves, they've made the longest dives in history,

travelling more than ten kilometres from the cave entrance.

They're sometimes underwater for 24 hours at a time.

Their efforts have revealed

one of the world's largest underwater cave systems.

It's part of a huge store of groundwater, of varying depths,

that underlies all of Florida and reaches into neighbouring states.

And it's not just the USA.

There's groundwater in the most unexpected places.

More than 3o% of all the fresh water on Earth is under our feet.

Looked at this way, our apparently solid planet is more like a sponge.

In our early history,

the need for reliable supplies of water led us to rivers and groundwater.

But as humans spread across the planet, they learned to exploit

the vagaries of the water cycle in many different ways.

The key was adaptation.

(THUNDER RUMBLES)

Take rain.

A familiar occurrence in many parts of the world.

But this is rain at its most extreme -

the monsoon.

The significance of the monsoon isn't the human discomfort

but how the people here have learned to liv