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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.
It has shaped the fate of entire continents.
..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 40 degrees Celsius,
and there's less than a centimetre of rainfall each year.
The whole thing's moving.
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 6,000 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.
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 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 6,000 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.
6,000 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.
10,000 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 5,500 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)
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.
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 1,000th 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.
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.
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.
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 12,000 years ago.
It all began with a big block of ice.
12,000 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 12,500 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 -
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...
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,
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.
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 -
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.
140 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 -
5,000 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.
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 2,500 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 2,000 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...
...40 to 50 metres - that's about 150 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 50 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 30% 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.
A familiar occurrence in many parts of the world.
But this is rain at its most extreme -
The significance of the monsoon isn't the human discomfort
but how the people here have learned to live with it.
I'm travelling to the very epicentre of the monsoon,
a place called Cherrapunjee,
which holds the world record for the highest rainfall in a single year.
I thought I knew rain.
If you're from the west of Scotland, you've met rain before,
but this is different, it's different rain.
It's hard to explain. It's the sheer intensity of it -
it just comes barrelling down.
But also, the raindrops are massive.
You feel as if you could fill an egg cup with them, which means that,
within minutes, you're just soaked.
It's pointless with a hood and all the rest of it - I'm soaked.
What I really need is a brolly, like this chap.
Just watch it, it's very slidy.
Back in west Scotland, where I'm from, the average annual rainfall
is nearly a metre, and that might horrify a Californian,
but here in Cherrapunjee, the annual average rainfall
is more than ten times that - between 11 and 12 metres.
That's nearly the height of a four-storey building.
Streams turn to rivers, and rivers turn to torrents.
When you live with so much water, you have to adapt...
just to get around.
And that's exactly what the local Khasi people have done.
Look at this!
Isn't this fantastic? Look at it!
It's a living bridge - look, you can see all these roots coming down.
The texture of them is beautiful.
I mean, this entire structure is built of growing rubber tree.
It's just mad when you follow it!
You can see that this is the perfect union of the tree and the villagers.
The locals have kind of trained the roots,
kind of guided them through, knitted them together.
What they've done here is they've grabbed some rootlets like this
and taken it round. And look, here it is...
this set of rootlets here. That's incredibly strong.
It's an anchor for the bridge.
Ordinary bridges would rot under the relentless drenching of the monsoon.
What's clever about these root bridges is they get stronger as they get older.
So wide! I mean, a whole village could get through here.
Surprisingly, the intensity of the monsoon rain is all down
to a basic property of water.
Compared to other substances, water takes a lot of energy to heat up.
So the land and the ocean react very differently
to the rising temperatures of early summer.
During these months, India's land surface heats up much more
than the surrounding Indian Ocean.
The high temperature reduces the density of the air,
creating low pressure.
That sucks moist ocean air onto the land,
which brings rain.
It's because the whole system is driven by the sun's heat
that the rains come in the summer.
But it also means that the monsoon
only lasts for three months of the year.
For the rest of the time, there's virtually no rain.
(TRAIN HOOTER BLARES)
The people of India have adapted,
as much as they can, to these extremes of the monsoon.
I think it's this way.
It's great, you have to use your elbows in here.
But outsiders are not always so sensitive to its rhythms.
Here in India, the changing strength of the monsoon year on year
had really tremendous impacts on the country's political fortunes.
That's especially true of its recent colonial past,
the story of which was played out
against a backdrop of water abundance and scarcity.
Clearly there are lots of reasons to explain the fate
of British colonial rule in India,
but one of the least explored and most intriguing is water.
In the 19th century, the failure of the British
to manage India's water supply had significant consequences...
for them and for the Indian people.
Perhaps it was naivety, perhaps it was because they were outsiders,
perhaps it was their inability to cope with extreme weather,
but the British never really got to grips with the monsoon.
For thousands of years, people here have been developing ways
to deal with the monsoon.
And this was one of the most important - it's a huge open well
that was dug down deep enough to reach groundwater.
When the rains came, the water was filtered through the surrounding ground
and held in the well like a gigantic bucket.
But these stepwells, as they were known,
were more than water collectors.
The genius of this design was it turned the mundane need for water
into a social ritual.
People didn't just come here to dip for water - they gossiped,
they bathed, they even worshipped.
Over 3,000 stepwells were built, up until the 19th century.
For millions, they were the main source of water.
Despite the fact that structures like this helped the Indian people
survive droughts, the British didn't like it.
They were concerned that people bathing in the same water they drank from was bad news.
So on health grounds, they shut them down.
I mean, they may have had a point,
and they solved that issue by bringing in piped water,
but at the same time, they imported another problem
that was much, much worse.
It's a little-known fact,
but the British built canals on a colossal scale across India,
more than 57,000 kilometres of them -
perhaps their biggest engineering achievement anywhere.
Yet the British didn't realise that, even more than stepwells,
these huge bodies of standing water were a health hazard -
the perfect environment for malaria to breed and spread.
Given the lack of sensitivity the British showed to the Indian climate,
it's perhaps ironic that the monsoon played a significant role
in undermining British rule in India.
At the end of the 19th century, the monsoon rains failed.
For a decade, there were repeated droughts.
Crops were ruined, and there were terrible famines.
But the British failed to respond effectively -
in fact, they even continued to export rice.
This indifference to the rhythms of the monsoon fuelled popular anger
against colonial rule, and the independence movement grew rapidly.
Today, the stepwells are being repaired.
Pumps accessing groundwater are used to protect
against the unreliable monsoon.
And that's made India the largest user of groundwater in the world.
Adapting to the water cycle has meant the difference between success
and failure for many civilisations.
But there was another strategy that also brought success...
..and that was to take control of the water cycle.
There was one early civilisation above all others that took control
of the planet's most dramatically changing source of water.
They mastered the monsoon.
They were the Khmers,
and from the 9th century, they dominated the area
we now know as Cambodia.
And this was their greatest achievement...
the legendary temple complex of Angkor.
You get a real sense of the age of this place here,
cos this was built over 1,200 years ago.
In a few places, like here, you can see it's showing the age.
Look, the faces have all gone,
but, look at this, that looks as if it could have been carved just yesterday.
Angkor was built to honour the Hindu gods
and it symbolised the extraordinary success of the Khmers.
In a way, this place is a monument to something else -
the Khmers' ability to harness the power of the monsoon.
The Khmers were first drawn to this region
by the Tonle Sap lake and the river that feeds it.
Today, it's home to a floating, permanent community,
replete with all the necessary amenities.
All life here is lived on the river -
the whole village, houses, shops, churches, schools, everything.
A hardware store!
Everybody's watching telly. They're all watching soap operas,
or just chilling out.
People settle here today for the same reason
the Khmers did over 1,000 years ago -
the unusual behaviour of the lake around monsoon time.
Each year when the monsoon rains fall, the land around here
just can't drain fast enough, and this lake, Tonle Sap, swells enormously.
It more than trebles in size, becoming, for just a few months,
the largest freshwater lake in Southeast Asia.
And every year, the water brings with it a spectacular bounty.
Fish! Loads of them, nibbling away at your toes in this murky water.
So many, that when it floods, the Tonle Sap lake
becomes the richest source of freshwater fish in the world.
Back in the 9th century, the Khmers realised
that this annual influx of fish and water offered a glittering opportunity.
They set about building a fishing industry here, and with the profits,
they built the temples of Angkor.
But as it grew, the Khmer kingdom faced a stumbling block.
When the monsoon finished each year, the fish and water would vanish.
So each year, the inhabitants were plunged into drought and hunger.
The Khmer rose to the challenge magnificently.
They decided that rather than be at the whim of the monsoon,
they would make it work for them.
This is part of a vast network
of irrigation tunnels that crisscross the whole of Angkor.
When the Khmer started digging these in the 9th century,
people had seen nothing like them.
This was plumbing on a grand scale.
From the air, it's still visible today.
Over 1,000 years ago, the Khmers managed to divert a river
by 80 kilometres.
They built canals
that extended over an area of 1,000 square kilometres
and dug reservoirs
that could hold up to 600 million cubic metres of monsoon water.
With this system, the Khmers seized control of the planet's water cycle.
They turned the seasonal rainfall of the monsoon into a reliable,
all-year-round water supply.
It was an enormous achievement, enabling Angkor at its peak
to support a population in excess of one million.
Thanks to their control of water,
the Khmers had built the largest pre-industrial city in the world.
The Khmer hung on until the 15th century,
which was when the kingdom of Angkor finally went to the wall.
They were victims of their own success.
Their population went through the roof,
and they simply outstripped their resources, including -
despite all that incredible engineering -
including the water supply.
I guess that there are limits
to what even the mighty monsoon can sustain.
Today, we control water on a massive scale.
The world's reservoirs now hold over 10,000 cubic kilometres of water.
That's five times as much water as in all the rivers on Earth.
And because most of it is pooled in the more populated northern hemisphere,
away from the equator, the extra weight has slightly changed how the Earth
spins on its axis.
It's caused the Earth's rotation to speed up,
shortening the day by 8 millionths of a second in the last 40 years.
Today, we take our control of water for granted.
Modern civilisation couldn't exist without it.
But there's still only a finite amount of water to go around.
In many parts of the world,
scarcity has led to a bitter struggle for control over the available supply.
And that's true in even the wealthiest countries.
Today, Los Angeles is a city with every luxury and convenience.
Yet not so long ago, at the turn of the last century,
Los Angeles was struggling.
LA's problem was its location, hemmed in on three sides by desert
and on the fourth by ocean.
So it lacked the most basic requirement for city life -
a reliable water supply.
So it came up with a plan to get the water it so needed.
400 kilometres to the north of the growing city,
nestled within the Sierra Nevada mountain range,
was a place called Owens Valley.
It was a verdant place, where people were settling and building farms.
At the heart of it was plentiful water -
a wide river feeding a huge lake.
This valley must have seemed like the answer to Los Angeles' prayers.
There was enough water here to easily supply
over one million people.
There was only one problem...
it didn't belong to them.
It belonged to the farmers of Owens Valley.
It would have to be taken by stealth.
It wasn't long before men appeared in the valley,
masquerading as investors.
They offered to buy up farmland at seemingly irresistible prices,
just to get the water rights that went with it.
It wasn't technically illegal, but it was certainly shady.
And it worked.
In 1913, after six years of construction,
an aqueduct was opened.
And this is it.
In a way, this aqueduct was a triumph,
certainly as far as Los Angeles was concerned.
It allowed millions of people 200 miles down there
to live in a growing and vibrant city.
But that's not how people here saw it.
The Owens Valley farmers didn't give up without a struggle.
A kind of loose resistance movement started,
and they would take over places like this and open the sluice gates,
allowing the water to pour back down into Owens Valley.
And regularly they'd dynamite the aqueduct.
But the city rebuilt it,
and a game of cat and mouse continued
for three more dynamite-filled years.
Eventually, the police clamped down with a "shoot to kill" policy,
and the rebellion fizzled out.
The city had won.
Today, the Los Angeles Aqueduct is just part of a giant network
of pipes and aqueducts all serving one of the world's great cities.
But, back in Owens Valley, the lake has all but vanished,
and the river is barely a trickle.
The story of Owens Valley is not an isolated case.
Today, there are conflicts over water taking place all around the world.
Israel, the Palestinians, Syria and Jordan
dispute access to the River Jordan.
Egypt, Sudan and Ethiopia quarrel over the waters of the Nile.
On the Indus river,
India and Pakistan are in conflict over dams built on the river's tributaries.
And these are only some of the more well-known examples.
10,000 years ago, we lived at the whim
of the unpredictable water cycle.
Since then, we have harnessed the power of rivers
to advance our civilisations.
We have extracted groundwater from the depths
of the most unlikely places.
And we have learned to redirect and store water on a massive scale.
Today, we have unprecedented power over the planet's water.
But one thing hasn't changed -
there's still only a finite amount of water on Earth.
It seems to me
that water is the Achilles heel of our modern civilisation.
It's the one resource,
more than any other,
with the potential to limit our ambitions.
The fundamental limits of the water cycle are still there.
But the lesson of history is that the most successful civilisations
learn to adapt to those limits.
So the problem is more with us.
Now, that prospect may find you gloomy or, like me, more optimistic.
But either way, at least the future's in our hands.
Next time, wind. For thousands of years,
the wind has shaped the destiny of people
across the planet.
It's made fortunes
and brought ruin.
Even today, we're still at its mercy.
Professor Iain Stewart continues his epic exploration of how the planet has shaped human history.
This time he explores our complex relationship with water. Visiting spectacular locations in Iceland, the Middle East and India, Iain shows how control over water has been central to human existence.
He takes a precarious flight in a motorised paraglider to experience the cycle of freshwater that we depend on, discovers how villagers in the foothills of the Himalayas have built a living bridge to cope with the monsoon, and visits Egypt to reveal the secret of the pharaohs' success.
Throughout history, success has depended on our ability to adapt to and control constantly shifting sources of water.