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Of all our planet's forces,
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perhaps none has greater power
over us than water.
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For me, water's the most
magical force on Earth.
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The presence of water shapes,
renews and nourishes our planet.
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Oh, my gosh!
You're getting all wet there!
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It's our planet's lifeblood.
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It pumps through it continuously,
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delivering vital ingredients for life.
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Ah, it's glorious.
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Water makes Earth alive.
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Yet water is just one of the ways
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that the power of the planet
has shaped our lives.
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The Earth has immense power...
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...and yet that's rarely mentioned
in our history books.
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I'm here to change that.
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In this series, I'm exploring
four great planetary forces
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that have influenced our history.
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The power of the deep Earth...
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...that fuelled technological innovation.
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Wind.
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It has shaped the fate
of entire continents.
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And fire...
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...which gave us the power
to conquer the planet.
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But I'm going to start with water.
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The magic of water is that
it's constantly transforming itself,
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shifting between guises
and from place to place.
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Our struggle to control it
has been behind the rise and fall
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of some of
the greatest civilisations on Earth.
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The centre of the Sahara Desert
in North Africa.
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One of the driest places on Earth.
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I'm over six hours'drive
from civilisation.
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Temperatures here regularly reach
4o degrees Celsius,
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and there's less than a centimetre
of rainfall each year.
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Ah....
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The whole thing's moving.
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(HE STRAINS)
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It's like walking on water.
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Yet hidden amongst these dry dunes
are clues
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that point to the dramatic influence
the planet has had on human lives.
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I've come here because although
you'd never know it,
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the story of this place
is all about water.
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The clues are etched
into that rock face there.
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Prehistoric rock art
dating back 6,OOO years,
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and depicting the most unlikely
cast of characters you've ever seen.
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Wow, what is that?
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It's a giraffe...
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It's a giraffe, look at it,
there's the neck.
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There's its ears, that's an eye,
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and its mouth.
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That's really natural, isn't it?
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00:04:30,389 --> 00:04:32,949
And this looks like the giraffe
dipping its head down,
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drinking some water -
we've got a herd of giraffes here!
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There's two cats.
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They're fighting.
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This... What is this?
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It looks like the figure of a man,
but he's wearing a bikini.
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00:05:05,509 --> 00:05:09,343
And this is clearly a crocodile,
which is especially odd here.
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This is an aquatic animal,
it doesn't just paddle around.
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It needs a lot of water to live in.
In fact,
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all the creatures that are depicted
on these rocks are not desert animals -
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they need wet conditions.
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In such a parched wilderness,
how can this be?
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The only explanation is that
6,ooo years ago, this place was wet.
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Once you know what to look for,
the evidence is all around.
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Up there is a river valley
that's been carved out into the rock,
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and it's been carved by running water
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which has flowed down here,
smoothing off this rock bed,
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and then cascaded down into the valley
and off there.
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6,OOO years ago,
that was a big river.
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Satellite images reveal
that the river bed I'm standing in
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is just one of a network
of past river valleys
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that crisscross the Sahara Desert.
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1 o,ooo years ago, this dry, empty place
was entirely different.
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Little is known about the early Saharans
who lived here then,
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but we do know that they depended
entirely on water.
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Water formed the lakes
in which they swam.
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Water nourished the plants
which fed the animals they hunted.
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Water filled the clay pots
from which they drank.
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But then the climate changed.
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About 5,5OO years ago,
the Sahara began to dry.
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The rains failed, the rivers shrank,
and the lakes dried out.
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For the early Saharan people
there was only one option -
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to follow the rains
and abandon the desert.
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The fortunes
of the early Saharan people
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reveal a universal, timeless truth -
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our fate is inextricably linked to water.
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The problem is,
water never stands still.
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It's always on the move
across the planet.
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00:08:05,189 --> 00:08:07,578
We think of this as a blue planet.
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But while water is abundant,
most of it is no use.
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More than 97% of the Earth's water
is salty ocean, which we can't drink
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or use to grow crops.
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Less than 3% is fresh water,
on which all human life hangs.
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What's more, that tiny fraction
is often hard to pin down,
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because fresh water
has a life cycle all of its own.
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I'm about to explore that cycle,
in all its elusive glory.
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You know, water seems so familiar,
doesn't it?
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But to see its remarkable qualities
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you have to go
to some extreme lengths.
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(MOTOR CHUGS INTO LIFE)
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(REWING)
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Here we go...
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Ho-ho! Feel that!
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( # WAGNER: Ride Of The Valkyries)
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Here we go!
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Oh... Hey-hey! Oh, we're off!
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Oh, my God!
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It's a bit bouncy!
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I shouldn't have had
that bacon and eggs this morning.
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O-o-o-h! (LAUGHS)
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The fresh water that we depend on
begins its life in the oceans.
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As the sun's rays beat down
on the surface of the sea,
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they heat the water molecules
until some evaporate.
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It's the start
of an extraordinary journey.
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You know, when water evaporates,
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it feels as if it vanishes into thin air.
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But although we barely notice it,
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water molecules are suspended
around us all the time.
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It's just that we're only aware of it
when they clump together
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as cloud.
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At any one time, less than a thousandth
of the world's fresh water is up here
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in the atmosphere.
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It may not seem much, but this is what
spreads water from the seas to the land.
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A water molecule doesn't hang around
up here for very long.
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In fact, it spends less time
up here in the atmosphere
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than at any other time on its journey -
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a mere nine days
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until the typical water molecule
crashes to Earth as rain.
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(THUNDER RUMBLES)
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(BIRD SQUAWKS)
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For most of us, rain is perhaps
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the most familiar stage
of the water cycle,
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but notoriously the least reliable.
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As the water falls as rain,
it joins a bigger system,
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cascading and carving its way across
the land surface as streams and rivers.
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Look at that!
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Water absolutely everywhere!
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Rivers and rain are the part
of the water cycle that we depend on.
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Whoo-hoo!
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And yet they're only a tiny proportion
of the world's fresh water...
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00:11:59,669 --> 00:12:04,379
...a measly 2%
of all fresh water on the planet.
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00:12:04,429 --> 00:12:07,023
The rest of the Earth's fresh water
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is locked away down there,
on the ground.
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Oh... (LAUGHING)
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Oh!
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What a landing!
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The vast majority of it is stored as ice.
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Most of the rest
seeps deep into the Earth,
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where it's known as groundwater.
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Hidden away down here
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is the planet's second-largest store
of fresh water.
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00:12:42,829 --> 00:12:46,868
But in the end,
all water arrives back in the oceans,
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and the cycle begins again.
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What that circulation
means for us humans
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is that water is a moving target.
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We constantly have to seek it out
on its endless cycle and intercept it
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wherever and whenever we can.
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This quest to...
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to pin down water has played
a defining role in human history.
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You can trace the impact
of our quest for water
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right back to the dawn of civilisation,
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about 1 2,ooo years ago.
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It all began with a big block of ice.
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1 2,ooo years ago,
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much of the northern hemisphere
was covered in a single, huge ice sheet.
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00:13:49,989 --> 00:13:52,628
And even today
you can see its legacy...
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...here in Iceland.
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This glacier is a tiny remnant
of that once enormous expanse of ice.
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00:14:19,229 --> 00:14:24,747
Ice is like a storage cupboard in the
circulation of water around the planet,
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a store into which water
can be deposited or withdrawn.
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And it was a shift in the amount
of water locked up here
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that was to drive one of the greatest ever
transformations of human society.
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Today, the ice sheet here
is melting and retreating,
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and releasing
this great armada of icebergs.
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00:14:49,909 --> 00:14:52,628
But if you go back 1 2,5OO years ago,
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it's a very different story.
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Then the ice was expanding,
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sucking moisture out of the atmosphere
in vast quantities
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and locking it away in the ice.
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And the effects of that
were felt right across the planet.
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Thousands of kilometres
away in the Middle East...
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...it led to a drought
which lasted for centuries.
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It had its most profound impact
in what would become known
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as the Fertile Crescent, an area famed
for its exceptionally rich soil.
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This drought would trigger the start
of the defining characteristic
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of human civilisation.
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Back then, every human on the planet
was a hunter-gatherer.
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Those living in the Fertile Crescent,
the Natufians, thrived on rich pickings
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of fruit and berries,
with plenty of deer and ibex to hunt.
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00:16:13,869 --> 00:16:18,738
But as the drought took hold,
to survive they would have to adapt.
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00:16:20,549 --> 00:16:24,542
They came up
with two distinct strategies.
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00:16:24,589 --> 00:16:28,787
One group developed this,
the Harif point,
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00:16:28,829 --> 00:16:31,104
a new, state-of-the-art arrowhead
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that allowed them to tackle a drought
by hunting more efficiently.
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00:16:35,149 --> 00:16:39,188
But a second group came up
with something a little bit more subtle.
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00:16:39,229 --> 00:16:43,142
Although you wouldn't know it,
this is a sickle,
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00:16:43,189 --> 00:16:47,546
and it offered a completely new approach
to gathering food.
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00:16:47,589 --> 00:16:51,264
This small, stone blade represented
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00:16:51,309 --> 00:16:56,064
a decision not to chase food,
but to stay put and grow it.
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00:16:59,389 --> 00:17:02,301
The Harif point did a good job
for the hunters.
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00:17:04,589 --> 00:17:08,298
But it was the sickle
that really changed history.
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00:17:09,589 --> 00:17:13,468
In a drought,
it's safer to stay close to water,
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00:17:13,509 --> 00:17:19,027
but that decision to remain in one place
meant planting crops was essential.
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00:17:19,069 --> 00:17:21,822
If you go foraging in the forest,
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00:17:21,869 --> 00:17:24,702
you can only collect so much food
with your bare hands,
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00:17:24,749 --> 00:17:28,458
but if you've got one of these,
you can harvest fast and furious,
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00:17:28,509 --> 00:17:32,058
and for the same amount of effort,
you can collect far more food.
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00:17:32,109 --> 00:17:34,498
With this simple tool,
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00:17:34,549 --> 00:17:38,098
these people had begun
the agricultural revolution.
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00:17:38,149 --> 00:17:41,858
And the rest, as they say,
is history.
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00:17:45,349 --> 00:17:49,627
A lack of water
and a simple but ingenious response
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00:17:49,669 --> 00:17:52,183
led to the birth of civilisation.
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00:17:55,869 --> 00:17:59,305
But once farming took hold,
it had a profound impact
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00:17:59,349 --> 00:18:01,260
on our relationship with water.
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00:18:03,029 --> 00:18:06,908
No longer
could we simply follow the rains.
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00:18:06,949 --> 00:18:11,022
Now people needed
regular, reliable sources of water
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00:18:11,069 --> 00:18:13,424
to make sure their crops grew.
217
00:18:15,549 --> 00:18:19,178
So the need for water
began to define
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00:18:19,229 --> 00:18:21,902
where the first civilisations
could flourish.
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00:18:23,949 --> 00:18:27,259
That led people to the one stage
of the water cycle
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00:18:27,309 --> 00:18:30,187
that offers reliable fresh water -
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00:18:30,229 --> 00:18:32,265
rivers.
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00:18:33,989 --> 00:18:35,342
Across the planet,
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00:18:35,389 --> 00:18:38,301
rivers cover a tiny proportion
of the Earth's surface,
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00:18:38,349 --> 00:18:41,978
but for the first farmers,
they became magnets.
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00:18:47,469 --> 00:18:50,461
But rivers did more than supply
a steady source of water.
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00:18:50,509 --> 00:18:54,218
They changed the very character
of the civilisations
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00:18:54,269 --> 00:18:56,578
that grew up along them,
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00:18:56,629 --> 00:19:01,828
influencing everything
from politics to social organisation.
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00:19:04,749 --> 00:19:09,459
The power of rivers to shape history
is graphically illustrated
230
00:19:09,509 --> 00:19:13,184
by perhaps the greatest
of all early civilisations...
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00:19:19,509 --> 00:19:21,784
...Ancient Egypt.
232
00:19:23,149 --> 00:19:26,027
You might think you know the story -
233
00:19:26,069 --> 00:19:28,981
a mighty civilisation
that built the pyramids
234
00:19:29,029 --> 00:19:33,102
under the autocratic rule
of ruthless Pharaohs.
235
00:19:35,149 --> 00:19:38,744
But if you want to understand
what really made Egypt great,
236
00:19:38,789 --> 00:19:43,146
you have to leave the pyramids
and the temples behind...
237
00:19:46,029 --> 00:19:51,820
...and come here, to a small place
that hardly anyone visits.
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00:19:51,869 --> 00:19:55,782
You know, at first glance
these look like your average, everyday,
239
00:19:55,829 --> 00:19:58,343
2,OOO-year-old steps.
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00:20:00,509 --> 00:20:04,980
But this staircase
is what made Ancient Egypt tick.
241
00:20:05,029 --> 00:20:09,341
You get an idea of its true purpose
by the markings on the side wall -
242
00:20:09,389 --> 00:20:12,984
these grooves were carefully carved
into the marble -
243
00:20:13,029 --> 00:20:17,341
because this was
a beautifully simple measuring device.
244
00:20:17,389 --> 00:20:21,223
And to see what it was measuring,
you have to pop round the corner.
245
00:20:23,909 --> 00:20:25,467
Oh!
246
00:20:25,509 --> 00:20:26,908
It's all wet!
247
00:20:29,069 --> 00:20:31,663
And this is it -
248
00:20:31,709 --> 00:20:33,301
the Nile river.
249
00:20:33,349 --> 00:20:37,820
That set of steps and markings
is a Nilometer.
250
00:20:37,869 --> 00:20:41,623
It measured the changing level
of the river.
251
00:20:41,669 --> 00:20:45,708
Each year when it flooded, the maximum
height that the waters came to
252
00:20:45,749 --> 00:20:48,627
would directly predict
the yield of the crops
253
00:20:48,669 --> 00:20:51,627
and, with that,
the profits that the farmers made.
254
00:20:54,869 --> 00:20:56,905
It worked
because the water of the river
255
00:20:56,949 --> 00:20:59,986
carried something special within it -
256
00:21:00,029 --> 00:21:01,860
an almost invisible treasure
257
00:21:01,909 --> 00:21:06,380
that was the secret
of Egypt's economic might.
258
00:21:07,949 --> 00:21:11,385
What made Egypt great is this stuff -
259
00:21:11,429 --> 00:21:13,021
silt.
260
00:21:13,069 --> 00:21:17,221
It's a rich soup of minerals, which...
261
00:21:17,269 --> 00:21:19,419
It's like an espresso.
262
00:21:19,469 --> 00:21:23,667
Tiny flecks of rock and minerals
that the river picked up
263
00:21:23,709 --> 00:21:28,100
over its wandering course
and swept along with the flow.
264
00:21:36,029 --> 00:21:38,543
All rivers carry some silt,
265
00:21:38,589 --> 00:21:41,661
but the Nile has the benefit
of starting in Ethiopia,
266
00:21:41,709 --> 00:21:45,258
where the rock is young and volcanic.
267
00:21:46,309 --> 00:21:49,267
This forms the richest of silts.
268
00:21:49,309 --> 00:21:54,224
1 4o million tonnes of the stuff
are carried by the Nile down river
269
00:21:54,269 --> 00:21:56,703
to Egypt each year.
270
00:21:56,749 --> 00:21:59,582
Every year, the seasonal flood
covered the fields
271
00:21:59,629 --> 00:22:05,499
and left behind nutrient-rich silt
that fertilised the crops.
272
00:22:05,549 --> 00:22:09,622
The more silt,
the more food was produced.
273
00:22:09,669 --> 00:22:11,785
It was the size of the flood -
274
00:22:11,829 --> 00:22:16,857
and with it the bounty of silt -
that the Nilometer was used to predict.
275
00:22:18,949 --> 00:22:24,023
So, simply by measuring the height
of the Nile, the Egyptians were able
276
00:22:24,069 --> 00:22:28,585
to forecast food production
and, with it, the profits of the farmers.
277
00:22:30,149 --> 00:22:34,301
Each year, they used this information
to set tax levels.
278
00:22:34,349 --> 00:22:39,184
So the wealth and the might
and the splendour of Ancient Egypt
279
00:22:39,229 --> 00:22:43,108
is all down to a simple twist
of geographical fate.
280
00:22:43,149 --> 00:22:46,664
In fact, Ethiopia itself gets
almost no benefit
281
00:22:46,709 --> 00:22:49,860
from that fertile soil
washed from its highlands.
282
00:22:49,909 --> 00:22:53,618
It's even said that
its greatest export is the silt
283
00:22:53,669 --> 00:22:58,618
that it sends down the Nile,
silt that made the Pharaohs rich.
284
00:23:01,309 --> 00:23:04,779
But the ebb and flow of the Nile
285
00:23:04,829 --> 00:23:09,380
had more far-reaching implications
for the Egyptian people than mere taxes.
286
00:23:15,349 --> 00:23:20,139
Intriguingly, it may be that
where access to water is limited,
287
00:23:20,189 --> 00:23:22,305
that actually determines
288
00:23:22,349 --> 00:23:27,025
the way a society is organised
and even its use of slavery.
289
00:23:30,789 --> 00:23:33,144
Where water is in short supply -
290
00:23:33,189 --> 00:23:36,101
or from a single source,
as it is in Egypt -
291
00:23:36,149 --> 00:23:41,860
then you need a highly structured
society to get the best out of it.
292
00:23:45,069 --> 00:23:48,459
For large-scale irrigation,
you need bureaucrats to decide
293
00:23:48,509 --> 00:23:50,181
where to dig the water channels.
294
00:23:50,229 --> 00:23:53,062
You need teams of working men -
slaves, really -
295
00:23:53,109 --> 00:23:55,100
to do the actual hard work of digging.
296
00:23:55,149 --> 00:23:59,461
And once the channels are in place,
you need farmers with money enough
297
00:23:59,509 --> 00:24:01,579
to buy the water it's delivered.
298
00:24:01,629 --> 00:24:04,701
So right away you've got
three tiers of society,
299
00:24:04,749 --> 00:24:07,582
and I haven't even mentioned
the Pharaohs.
300
00:24:09,269 --> 00:24:13,501
So the rigid, hierarchical structure
of Egyptian society
301
00:24:13,549 --> 00:24:17,144
wasn't just dictated by the Pharaohs.
302
00:24:17,189 --> 00:24:21,228
It also emerged because the Egyptians
had only one water source -
303
00:24:21,269 --> 00:24:22,748
the Nile.
304
00:24:28,589 --> 00:24:30,466
5,ooo years ago,
305
00:24:30,509 --> 00:24:34,343
it wasn't just the Ancient Egyptians
who noticed the value of rivers.
306
00:24:36,229 --> 00:24:41,383
Other great civilisations were also
forming along the banks of rivers.
307
00:24:42,949 --> 00:24:44,780
In Mesopotamia,
308
00:24:44,829 --> 00:24:50,028
the Sumerian civilisation flourished
between the Tigris and the Euphrates.
309
00:24:51,949 --> 00:24:57,262
Further east, the Harappan civilisation
formed by the Indus.
310
00:24:59,149 --> 00:25:04,348
And early Chinese civilisations
were emerging along the Yellow River.
311
00:25:08,629 --> 00:25:12,941
But not all early farmers were content
to settle by rivers.
312
00:25:12,989 --> 00:25:15,901
Others learned
to exploit new sources of water,
313
00:25:15,949 --> 00:25:18,179
in the unlikeliest places.
314
00:25:20,309 --> 00:25:23,267
Like the Sahara Desert, in Libya.
315
00:25:32,589 --> 00:25:36,264
These are the remains
of the ancient city of Garama,
316
00:25:36,309 --> 00:25:41,702
which about 2,5oo years ago
was the centre of a powerful empire.
317
00:25:41,749 --> 00:25:45,458
Today, it's a bit of a maze,
but from up here
318
00:25:45,509 --> 00:25:49,707
you can see the shapes of the buildings,
the way the streets interconnect.
319
00:25:55,629 --> 00:25:59,622
You get a real sense of how this place
must have worked in its prime.
320
00:26:08,989 --> 00:26:11,708
This was the home
of the Garamantians...
321
00:26:13,269 --> 00:26:16,420
...which, for me,
are a rather forgotten people.
322
00:26:16,469 --> 00:26:20,462
They've been eclipsed in the history books
by their showy contemporaries,
323
00:26:20,509 --> 00:26:22,306
the Greeks and the Romans.
324
00:26:22,349 --> 00:26:28,140
The Garamantians dominated
the Sahara Desert for almost 2,OOO years.
325
00:26:28,189 --> 00:26:32,819
They were the society that first brought
civilisation to the desert.
326
00:26:36,269 --> 00:26:40,740
Far from just scraping by
in this harsh landscape,
327
00:26:40,789 --> 00:26:43,428
the Garamantes were flourishing.
328
00:26:45,069 --> 00:26:48,618
They grew crops
such as cereals and grapes.
329
00:26:48,669 --> 00:26:50,978
They kept horses and pigs.
330
00:26:51,029 --> 00:26:54,339
Clearly, they needed
large amounts of water.
331
00:26:57,029 --> 00:27:00,658
So where did they find it,
here in the middle of the desert?
332
00:27:04,509 --> 00:27:09,185
Now, this is the key to the Garamantians'
incredible success.
333
00:27:09,229 --> 00:27:13,063
It's vertical holes
that are sunk deep into the ground...
334
00:27:14,509 --> 00:27:20,186
...4O to 5O metres -
that's about 1 5O feet.
335
00:27:20,229 --> 00:27:22,106
And the purpose of them
was pretty simple -
336
00:27:22,149 --> 00:27:24,617
it was to bring water up
from below ground.
337
00:27:24,669 --> 00:27:28,344
In this environment, it must have
seemed like it was almost magic.
338
00:27:28,389 --> 00:27:32,701
In fact, the Garamantians
had discovered groundwater.
339
00:27:34,589 --> 00:27:38,582
Beneath the surface of the Sahara
is a surprising part
340
00:27:38,629 --> 00:27:40,540
of the great water cycle -
341
00:27:40,589 --> 00:27:43,661
a massive store of groundwater.
342
00:27:47,429 --> 00:27:50,501
This is water
that has seeped into the ground
343
00:27:50,549 --> 00:27:54,383
and has collected
in porous layers of rock.
344
00:27:54,429 --> 00:27:56,659
The water came from the period
345
00:27:56,709 --> 00:28:01,942
thousands of years before,
when the Sahara was lush and wet.
346
00:28:03,509 --> 00:28:08,378
Some of that water percolated
into the rocks below and remained there,
347
00:28:08,429 --> 00:28:11,387
despite the dramatic drying above...
348
00:28:12,989 --> 00:28:17,062
...until the Garamantes found it.
349
00:28:17,109 --> 00:28:19,987
You kind of dig them down
until you hit the water table
350
00:28:20,029 --> 00:28:22,338
and then you just keep doing
the same thing.
351
00:28:22,389 --> 00:28:26,985
There's one after another, after another,
all in a whole line.
352
00:28:28,149 --> 00:28:32,267
But these holes aren't wells -
they're maintenance shafts.
353
00:28:32,309 --> 00:28:36,302
They reach down to tunnels
which carried the water.
354
00:28:36,349 --> 00:28:40,262
The point is, right up there at the end
is where the water source is,
355
00:28:40,309 --> 00:28:44,541
so the water flows naturally from
the escarpment up there, underground,
356
00:28:44,589 --> 00:28:47,387
down to the kind of oasis over there.
357
00:28:47,429 --> 00:28:50,068
Now, that's where
the Garamantians' city was.
358
00:28:50,109 --> 00:28:53,465
What they could have done is
they could have dug wells down
359
00:28:53,509 --> 00:28:57,946
and lifted the water out, but that's
a lot of work for very little return.
360
00:28:57,989 --> 00:29:01,186
Much better to use gravity
to channel the water
361
00:29:01,229 --> 00:29:04,187
in an underground tunnel
straight to where they need it.
362
00:29:05,749 --> 00:29:09,378
That was the Garamantians'
real ingenuity.
363
00:29:13,349 --> 00:29:17,422
The Garamantes had managed
to tap the same water
364
00:29:17,469 --> 00:29:22,145
that the early Saharans had enjoyed
thousands of years earlier.
365
00:29:22,189 --> 00:29:25,625
By mining groundwater,
the Garamantians managed
366
00:29:25,669 --> 00:29:30,868
to turn the clock back on the Sahara -
they made the desert bloom again.
367
00:29:33,309 --> 00:29:36,619
But the human struggle
to pin down water
368
00:29:36,669 --> 00:29:39,467
is forever balanced on a knife edge.
369
00:29:39,549 --> 00:29:43,224
Get that balance wrong
and you pay the price.
370
00:29:44,269 --> 00:29:50,902
For all their ingenuity, the Garamantes
over-exploited their groundwater.
371
00:29:50,949 --> 00:29:56,103
Eventually it ran out,
and so did their civilisation.
372
00:29:57,469 --> 00:30:00,905
Now all that remains are the bats.
373
00:30:00,949 --> 00:30:02,860
(SQUEAKING AND FLUTTERING)
374
00:30:07,069 --> 00:30:12,189
Today, modern Libyans have tapped
into this same groundwater supply,
375
00:30:12,229 --> 00:30:16,939
by using pumps to reach deeper
than the Garamantes could.
376
00:30:16,989 --> 00:30:22,507
But just like their ancient predecessors,
they're exploiting a finite resource.
377
00:30:22,549 --> 00:30:26,747
At most,
it will last only another 5o years.
378
00:30:29,029 --> 00:30:33,580
But water in this most inaccessible stage
of the water cycle
379
00:30:33,629 --> 00:30:35,859
is found in many other places.
380
00:30:37,429 --> 00:30:41,820
It's at its most spectacular
in Tallahassee, in Florida.
381
00:30:43,909 --> 00:30:49,188
Here, divers are just beginning
to explore a mysterious series of caves
382
00:30:49,229 --> 00:30:55,259
called a karst system, carved out
by groundwater over millions of years.
383
00:30:58,109 --> 00:31:02,546
This is one of the planet's
least known frontiers.
384
00:31:03,589 --> 00:31:06,262
When they began,
these divers had no idea
385
00:31:06,309 --> 00:31:08,265
of the extent of the cave network.
386
00:31:11,669 --> 00:31:16,060
To explore these caves,
they've made the longest dives in history,
387
00:31:16,109 --> 00:31:19,863
travelling more than ten kilometres
from the cave entrance.
388
00:31:22,669 --> 00:31:27,379
They're sometimes underwater
for 24 hours at a time.
389
00:31:32,189 --> 00:31:34,419
Their efforts have revealed
390
00:31:34,469 --> 00:31:38,178
one of the world's largest
underwater cave systems.
391
00:31:39,749 --> 00:31:44,220
It's part of a huge store of groundwater,
of varying depths,
392
00:31:44,269 --> 00:31:49,707
that underlies all of Florida
and reaches into neighbouring states.
393
00:31:49,749 --> 00:31:52,217
And it's not just the USA.
394
00:31:52,269 --> 00:31:56,945
There's groundwater
in the most unexpected places.
395
00:31:56,989 --> 00:32:01,858
More than 3o% of all the fresh water
on Earth is under our feet.
396
00:32:04,069 --> 00:32:10,907
Looked at this way, our apparently
solid planet is more like a sponge.
397
00:32:18,189 --> 00:32:20,100
In our early history,
398
00:32:20,149 --> 00:32:25,621
the need for reliable supplies of water
led us to rivers and groundwater.
399
00:32:28,789 --> 00:32:33,419
But as humans spread across the planet,
they learned to exploit
400
00:32:33,469 --> 00:32:37,144
the vagaries of the water cycle
in many different ways.
401
00:32:41,269 --> 00:32:44,022
The key was adaptation.
402
00:32:45,589 --> 00:32:47,580
(THUNDER RUMBLES)
403
00:32:48,589 --> 00:32:50,739
Take rain.
404
00:32:55,309 --> 00:32:59,666
A familiar occurrence
in many parts of the world.
405
00:32:59,709 --> 00:33:03,224
But this is rain
at its most extreme -
406
00:33:03,269 --> 00:33:05,066
the monsoon.
407
00:33:14,309 --> 00:33:18,268
The significance of the monsoon
isn't the human discomfort
408
00:33:18,309 --> 00:33:22,621
but how the people here
have learned to liv34952
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