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>> Earth, a unique planet,

restless and dynamic.

Continents shift and clash,

volcanoes erupt.

Glaciers grow and recede,

titanic forces

that are constantly at work,

leaving a trail of geological

mysteries behind, none greater

than the Sahara desert--the

hottest place on the planet, a

deadly wasteland that time

forgot, or so scientists

believed till they unearthed a

series of startling clues--the

fossils of sea creatures,

freshwater shells buried in

sand, ancient settlements with

human remains--clear evidence

that this stark landscape hides

a turbulent past, one that would

alter the course of human

history and provide a dramatic

new chapter in the story of how

The Earth Was Made. S02x04 Sahara Original Air Date on December 15, 2009

-- Sync, corrected by elderman -- -- for www.MY-SUBS.com --

Africa's vast Sahara desert is

as big as the United States.

The sand alone from this giant

expanse could bury the entire

world 8 inches deep.

It's the largest desert and the

hottest place on Earth.

>> When I first arrived in the

Sahara, I was just struck by how

utterly barren it was.

It's like the color green was

removed from the palette when

they made this place. Just

nothing, grays and browns and

not a scrap of life.

>> Smith's mission is to unearth

evidence of what made the Sahara

into the wasteland it is today.

Her investigation begins on the

desert's far eastern edge in

Egypt, not in the sands of the

Sahara, but in one of the most

epic structures ever built by

man--the great pyramids.

The building blocks of this

ancient wonder house a

remarkable clue to the history

of this land from a time long

before the pharaohs even

existed.

>> So, taking a closer look at

these blocks making up the

Pyramids, there's actually these

gorgeous marine fossils in here.

Most obvious are these flat

disks, up to about an inch wide.

They're called nummulites, and

they're actually single-celled

organisms.

>> The name nummulites means

little coins in Latin.

They are some of the largest

single-celled

creatures to have ever existed.

In ancient times, these blocks

were cut from quarries across

the country and dragged to the

pyramid site.

Each block weighed 2 1/2 tons,

and it took two million of them,

and over 20 years, to build the

Great Pyramid,

and, incredibly, up to 40% of

each building block is made up

of the bodies of ancient sea

creatures like these.

What's even more important about

the marine nummulites is that

they date back 40 million years

and they only lived in water.

>> It's a piece of evidence that

this area, now desert, was once

underneath the ocean.

You think about the

construction of the pyramid, but

not necessarily about what it's

made of and these gorgeous

fossils.

>> The Sahara now is the world's

biggest dustbowl, but the tiny

nummulites fossils suggest that

it was once very different,

that perhaps there could have

been water in this barren

wilderness.

Smith heads further inland to

investigate this extraordinary

idea.

She travels to a remote desert

valley near where the pyramid

stones were quarried.

It's called Wadi al Hitan.

Spread out in the sand lie

hundreds of fossils, first

excavated in 1983, but these are

nothing like the tiny

nummulites.

>> That's a whale.

So, here's the backbone, the

vertebrae.

Here's some ribs, the shoulder,

part of the front fin, and there

are the jaws.

This guy definitely did not live

in a desert.

>> This incredible fossil is a

Dorudun, an ancestor of modern

whales, one that died out 36

million years ago.

>> So, based on the size of

these vertebrae and how much of

the animal was here, it was

probably about 21 feet long.

This guy tells us that we were

underwater.

We were in the middle of an

ocean.

>> Wadi al Hitan is Arabic for

Valley of the Whales.

This 12-mile

dip in the landscape has the

highest concentration of fossils

in the world.

Some were found in the desert

floor, others in the cliff

walls.

The count so far is 400.

Almost all are marine animals,

further evidence that the Sahara

was once covered by sea.

>> So, here we've got the fossil

of a baby whale.

You can see the lower jaw down

here with some teeth and the

shoulder, the backbone, some

ribs and all curled around.

Actually, the tail comes right

back to near the head.

>> As if finding whales in the

desert wasn't intriguing enough,

there's yet another mystery to

be solved.

A lot of the fossils are of very

young dorudons, like this one.

A delicate mesh of stone helps

explain why so many baby whales

died in this place.

>> Wow, so finding that whale

fossil told us we were in the

ocean.

These rocks acutally tell us a

lot more about what type of

ocean we were dealing with.

These are all fossilized

mangrove roots.

>> These roots would have been

below water.

The mangrove trees would have

risen above it.

>> Since trees don't grow out of

the deep ocean, we know that

this area was actually under

shallow ocean at the time these

rocks were deposited, something

probably that looked like the

Florida Everglades, where there

are mangroves growing now.

>> Smith has discovered the

shoreline of the ancient sea.

The shallowness of the water

could explain why there were so

many young animals here.

>> So, one idea--there's a bunch

of these baby whale fossils

found in this area, and

this would have been a

shallow protected bay that maybe

the whales came just to birth

their young.

It is absolutely incredible to

see a fossilized whale in a

place that, right now, gets less

than a millimeter of rainfall a

year.

It's as much convincing evidence

for geologic change as I can

imagine.

>> The pieces of the puzzle are

coming together to reveal the

Sahara's watery past.

>> So, 40 million years ago,

this desert would have been

covered in the middle of this

valley by a shallow bay,

probably a brilliant tropical

blue-green color.

The shoreline would have been

off along the horizon, some

mangrove trees in the shallowest

parts of it.

>> Inland would have been a

vibrant combination of

rainforest and swampland.

The whales would have been drawn

to this ancient shore because of

the plentiful supply of food,

but their ocean is about to

vanish.

Marine fossils found in

Europe and Africa are evidence

that this ocean stretched almost

halfway around the world and

connectedAasia to the Atlantic.

It's called the Tethys Sea, and

much of the Sahara was submerged

under it.

The mystery is how and when this

lush water world turned into the

barren wasteland we see today.

The answer lies not in climate

patterns, but in geology.

The entire African continent

is underpinned by a giant piece

of the Earth's crust.

It's called a tectonic plate,

and 40 million years ago, in

what is known as the Eocene Age,

it was on the move.

>> So, at Wadi al Hitan, we have

whales swimming around in this

Eocene Ocean.

All the while, the African plate

is moving to the north.

Africa collides with Europe,

closing the Tethys Sea, but the

African plate keeps moving

>> So we uplifted the northern

part of Africa,

and so the Tethys Sea recedes,

and we've got this whole area of

North Africa now emerged.

It's out on land.

>> The whales of Wadi al Hitan

are cut off and trapped in

smaller and smaller pools of

water.

The deadly Sahara has claimed

its first victims.

In the quest to discover the

history of the vast Sahara

Desert, geologists have so far

uncovered two important clues.

Sea fossils in the great

Pyramids of Egypt show these

building blocks were once

underwater.

Whale bones reveal that a sea

submerged much of the Sahara 37

million years ago.

As the forces of plate tectonics

pushed the Sahara out from under

the sea, it created a tropical

swamp.

In order to figure out what made

it into the wasteland visible

today, scientists have to

pinpoint the moment of its

birth, but the clues to this

mystery turn out to be hidden in

the last place anyone expected.

>> 20 million years ago, the

Sahara desert was a lush

tropical swamp.

Geologists are now piecing

together the story of the next

20 million years.

Today the Sahara lies in what is

known as the desert belt, a

region of dry air north of the

equator.

Here, strong winds clear the sky

of clouds and dry out the land

below.

It stretches through the Gobi

Desert in China and across the

deserts of the southwestern

United States.

The Sahara is the largest, and

yet geologists know next to

nothing about when it was

created.

>> What we have are just these

little bits and pieces, these

snapshots of what the Sahara was

like, because the wind blows

away a lot of our record, and

what isn't blown away is often

covered by sand.

So, it's kind of hard to find

the rocks we need to tell the

story we're trying to tell.

>> One of the few places that

shelters a clue to the Sahara's

birth is the white desert.

In this hauntingly beautiful

site, dramatic shapes have been

sculpted out of rock.

>> Oh, this is great.

It's got that mushroom shape.

>> These structures are called

Yardangs, and they are a kind of

hourglass that could help

measure the age of the desert.

>> This rock is another

piece of evidence that this

desert was once under the ocean.

It's actually a chalk made up of

billions of little marine

micro-organisms.

These chalks are actually really

easy to erode, so that's one

reason things are so beautifully

sculpted by the wind.

>> But the wind is a brutal

creator.

It picks up sand and hurls it at

the Yardang.

>> When wind scours or

sandblasts the rocks, this is

the characteristic shape that we

get, this mushroom shape,

narrower in the middle.

That's because the wind goes

faster as you move up from the

ground, so it can erode harder,

but it has less sand in it

since it picks up the sand from

the ground.

So, where we get the most

erosion, where the rock is

narrowest, is where we have the

best mix of fast wind and the

most sand.

>> 40 million years ago, the top

of the Yardangs formed part of

the solid sea floor, but the

wind picked up once the Sahara

turned to desert, and the

process of carving out these

shapes began.

Figuring out how long that took

could help pinpoint the age of

the desert.

>> It's hard to say precisely

how long it would take for the

wind to carve this all out.

Something like this about 15

feet high, soft rock, you're

looking at maybe only tens to a

few hundreds of thousand of

years, but to carve out the

whole oasis depression, you've

got to need at least a million

years.

>> But geologists suspect the

Sahara is older than a million

years.

In their search for a more

accurate date, they next turn to

its most iconic feature--

Sand dunes.

Here in the Sahara, sandstorms

kick up that can last for 4

days.

The sand is hurled across the

terrain.

Over hundreds of thousands of

years, it accumulates into dunes

that can tower 50 stories high.

Perhaps these mountains of sand

hold the secret of the desert's

great age.

>> As soon as the climate

becomes arid, you can start

building dunes, and if we want

to know how long that took, we

can try and date the dunes

themselves, but that's really

hard.

>> What's hard is that these

dunes are constantly shifting.

The wind that builds them also

blows them away, moving them an

average of 50 feet a year.

To get a precise age for the

desert, scientists need to

follow the sand to the end of

its journey.

>> Coarse sand travels slowly

and doesn't go all that far, but

the finer particles will

actually travel further, and so

dust-sized particles actually

can get carried out into the

Atlantic.

>> The Sahara is the largest

source of dust on the planet.

500 million tons of it ends up

in the Atlantic every year.

Some of it reaches as far as

Florida and creates spectacular

red sunsets, but much of it

settles on the ocean floor, a

treasure trove of information

about the Sahara's past.

>> What's wonderful about

working with the ocean sediments

is that they capture everything.

They're this very faithful

recorder of the sediments that

are raining down from the

surface.

>> In 1995, Demenocal drilled

down into the ocean floor

through layers of mud dating

back millions of years.

Each layer of sediment is like a

time capsule.

Shallow levels show plenty of

this dust blown over from the

desert.

So, Demenocal extracted deeper

core samples from over a million

years ago.

Still there was desert dust.

Finally, he dug down to a layer

that was laid down 3 million

years ago, and there the dust

finally stopped.

It had taken a voyage to the

ocean floor to uncover the

turning point from humid,

tropical landscape into searing

desert.

>> That's a pretty amazing

change.

You don't think of something as

large and expansive and fixed as

the Saharan desert as being

something capable of such

profound changes, and yet this

is what the geologic record was

telling us.

>> Demenocal had finally solved

the riddle of the sands.

The Sahara has been a desert

wasteland for 3 million years.

In the search to discover the

age of the Sahara, geologists

have unearthed two startling

clues.

Yardangs show that windblown

sand has been blasting across

the desert for at least one

million years.

Deep-sea

cores give a more exact date.

The Sahara first turned from

swamp to sand 3 million years

ago.

From that moment on, the Sahara

became the searing wasteland we

see today.

It seemed that geology alone

could explain the creation of

the world's largest desert.

Then a new radar fitted to the

space shuttle revealed a

striking clue that the desert

once harbored a slash of green

across its burning sands.

>> Booster ignition and liftoff.

>> In 1981, the space shuttle

ade a surprising discovery.

Using a new type of radar, NASA

took a 30-mile-wide

scan of the Sahara desert.

The radar pierced the sand to a

depth of 16 feet and revealed

what looked like a hidden

network of ancient waterways

crisscrossing the desert.

This find has stumped scientists.

3 million years ago, the Sahara

turned from rainforest into

desert.

Now it seemed that it had been

home to a lot of water at some

point in the following 3 million

years.

Researchers followed the NASA

images north into Tunisia, to

the edge of a deep depression in

the Sahara, the site of their

first clue.

>> This is what we're

looking for.

There's some quite intact

shells here.

This deposit is largely composed

of intact shells.

>> And these are not shells from

the ocean.

>> So, here we have half of a

cadastradurnam glorcum shell,

which is clear sign of

a freshwater lake.

This is just one example of what

must be millions of shells.

We must be somewhere near the

shoreline of a lake here.

>> Further shell deposits reveal

that this lake was giant, about

the size of West Virginia, but

the shells provide even more

remarkable evidence--the date

when this lake existed.

Carbon dating puts them at

90,000 years old and leads to

one conclusion.

>> To have all of these shells

here, we must have had a lot of

rain falling in the

vicinity of this lake and a

green Sahara.

>> Scientists fanned out across

the Sahara to investigate other

satellite images.

They searched for any dips in

its landscape that looked like

they once held a body

of fresh water.

>> So, what was really exciting

for me about those radar images

produced by NASA, we can

link this to the gps, and we're

finding evidence of lakes

throughout the desert, and some

of these lakes are massive.

>> 3 different lake locations

were confirmed by the presence

of freshwater shells.

The series of ancient lakes were

so large, they've been dubbed

Megalakes.

>> So, this is the far shore of

the megalake.

This is just one of many lakes

across the green Sahara.

>> When Drake added up the

evidence of all the lake

locations, he uncovered an

astonishing fact.

>> Ok, so if this is North

Africa here and this is the edge

of the Sahara desert--so we're

here in Tunisia with a megalake

here, and we know there's

another megalake here in

Southern Libya, an even larger

one here in Chad--if they all

reached their maximum size, they

would have covered 10% of the

Sahara.

>> That would have made them 3

times larger than the Great

Lakes.

What is now the biggest desert

on Earth was once home to some

of the planet's largest bodies

of fresh water.

Sudden changes in climate have

been connected to everything

from volcanic activity to

meteors hitting the Earth, but

climate researcher Peter

Demenocal had a hunch this

wasn't the first time it had

happened.

He turned to his archive of deep

ocean cores.

>> Our approach was to use

deep-sea

sediments as this continuous

tape recorder, if you will, of

past climate change in Africa.

>> By looking at the levels of

desert dust in cores dating back

hundreds of thousands of years,

he discovered the Sahara had

changed more than once.

>> When we first collected these

measurements, I really kind of

almost fell back in my chair

because what we saw was, there

are many switches like this in

the climate system.

>> To explain these regular

dramatic changes, Demenocal

looked beyond the Sahara to the

rotation of the Earth itself,

more specifically, small wobbles

in the Earth's orbit around the

Sun.

The theory is that the wobble

causes the Earth to tilt

slightly.

So, the monsoons which drench

Southern Africa today shift up,

pouring rain onto the dunes of

the Sahara.

Crucially, these wobbles occur

every 20,000 years.

>> So, there's this perfect

one-to-one

match between when Africa was

wet and the stage of the wobble

cycle, and this goes back

millions of years.

>> Each time the rain belt moves

up, the landscape is

transformed, and the desert

turns green.

>> To me the single most

impressive thing about the

Sahara is how small fluctuations

in something as simple as a

wobble in the Earth's orbit can

lead to these really just

totally dramatic changes

in the climate of a region

that's so large.

>> Scientists now had evidence

of how and why the Sahara turned

green.

They knew that giant lakes

covered much of the desert, but

they had no proof they were

connected.

Were these vast isolated rain

pools or part of an

interconnected river system, as

suggested by NASA's radar

images?

Nick Drake gets word that, in

2009, archaeologists have made

an important discovery that

supports the river theory--stone

tools found not far from the

site of the megalake in Tunisia.

The first step is to identify

the shoreline of the ancient

lake.

>> Ah, now, this looks good.

So, we got some freshwater

mollusks.

>> These tiny shells are a good

sign of the lake.

Now he searches for what may

have been drinking the water

when the megalake existed.

>> We have what looks like part

of a jawbone of a small

herbivore.

You can see 3 teeth running

along there.

Some sort of gazelle.

>> So, animals must have existed

here in the desert when it was

green.

>> Ok. So, we got a stone

tool here.

It's become blunted, and they've

retouched it with a lot of very

fine flakes off the edge.

The fact that we got stone

tools and we've got dead animals

suggests hunting, and this was

the animal that was being

hunted, and then we have water,

so people sitting around a water

hole waiting for animals to come

to drink and then killing them,

eating them, leaving them

behind.

>> It's an important find.

A stone-age

tool shows people lived on the

shore of this ancient lake, and

90,000 years ago was a turning

point not just in the story of

the Sahara, but in the history

of humanity itself.

We are all descended from one

group of people in East Africa,

the birthplace of humankind.

Sometime between 80,000 and

120,000 years ago, modern humans

started the long journey out of

Africa.

No one knows for certain what

route they took.

The prevailing view is that

the Sahara was impassable, so

humans left East Africa and

traveled to the coast, then

crossed a land bridge into the

Arabian peninsula, but Drake

suspects that if the megalakes

were fed by a river system, it

would have created a green

corridor across the burning

sands.

>> So, our ancestors could have

followed this river system, gone

round the lakes, followed the

next river system around the

next lake and the next river

system on the last lake, and

then they'd be in

North Africa, and it would be

simple for them to just move

out.

The lakes is good story.

The rivers plus the lake is a

very concrete story.

>> To find proof of his theory,

Drake heads to one of the few

areas in the desert where water

still flows--an oasis.

He finds a small spring that

shelters a valuable clue.

>> Caught something. Great.

A cichlid fish.

Nearly all cichlid fish are

found south of the Sahara

Desert.

This particular type of cichlid

is the only example north of the

Sahara desert.

>> This cichlid bears a strong

resemblance to its closest

relative that lives in lake

Tanganyika, but this lake is

almost 3,000 miles away on the

other side of the Sahara.

>> So, the big question is, how

did it get here?

And the most obvious answer is

that it swam across the green

Sahara.

Modern humans who could live in

lots of different types of

environments would have

presumably found it very easy,

a darn sight easier than this

fish.

>> These rivers and lakes were

not to last.

The door slammed shut on the

green corridor, but scientists

now know that the Earth's wobble

makes the Sahara like a

pendulum.

It goes from wet to dry every

20,000 years like clockwork.

The investigation has now revealed

two clues to how these wobbles

affected the Sahara.

Freshwater shells dating back

90,000 years prove the desert

was once covered by

giant, freshwater megalakes.

A cichlid fish, shows the Sahara

was crossed by a river

that created a corridor of life

across the sand.

But scientists still needed concrete

information about how rapidly

these changes occurred.

Then an Egyptian archaeologist

made a stunning discovery in the

Libyan desert--an eyewitness

account of the Sahara's last

switch, the most dramatic

climate change of the last

10,000 years.

>> Scientists piecing together

the history of the Sahara have

uncovered a remarkable story.

40 million years ago, it was

covered in ocean.

3 million years ago, the Sahara

turned to desert.

Since then, it has swung between

grassland and wasteland every

20,000 years.

Scientists now turn to the more

recent geological past, the last

10,000 years, to discover how

fast this giant desert can

change.

Investigators head to a valley

deep in the Libyan desert.

The first clue to unraveling the

mystery is a small circle of

stones.

>> This is essentially the

foundation of a hut.

It is unimaginable to see an

actual house structure right

there next to what is now

nothing.

>> Hassan's discovery is

striking evidence of human

habitation.

>> The house structure consists

of this circular foundation with

upright standing blocks which

are taken from the local

bedrock.

It would have made a

semicircular structure with

probably skins and branches, and

people would have used that as

a shelter.

>> Hassan believes these huts

could have housed a small

community of around 50 people.

Now he needs to know exactly

when they lived here.

>> Ostrich eggshell beads.

>> These little ostrich egg

beads are clearly human

handiwork.

>> They used the eggshells to

make ornamental beads cut into a

circle so a string can pass

through.

So, they string these

into bracelets or necklaces.

>> The eggshells the beads were

made from are also here and

provide the next clue.

>> The eggshells suggest, of

course, that there were

Ostriches, and that's quite

remarkable for this environment

to have animals like that.

>> This was no nomadic tribe,

but a settled farming community

rearing animals for food.

Hassan carbon-dated

the ostrich eggshell beads.

The result?

Just 7,000 years ago, the

deadliest desert on Earth was

home to both human and animal

life.

It's dramatic evidence of the

last burst of green in the

desert.

A dip in the desert floor

provides a clear sign that rain

from the monsoon fell here.

>> What we have here is

the evidence of

a deep lake with mud deposited.

>> This mud indicates a body of

water that could have supported

a settlement of people.

>> When the lake is deep, as we

can see from these layers here,

there would be a lot of

vegetation, a lot of animals,

and people would have had a very

good time.

>> At sites all across the

Sahara, scientists have

excavated similar evidence of

life--the remains of elephants

and gazelles, hippos and

crocodiles.

Remarkable cave paintings even

show people swimming.

elsewhere, human bones have been

found, carefully buried in what

were lakeside graveyards.

Analysis of these bones reveals

they date from between 10,000

and 6,000 years ago.

The question now for scientists

was how quickly the Sahara

changed from bountiful back to

bone dry.

Earlier, deep-sea

cores had provided evidence of

the moment the Sahara first

turned to desert 3 million years

ago and of how, since then, a

wobble in the Earth's axis has

made it swing like a pendulum

between desert and grassland.

Now climate researcher Peter

Demenocal is on the hunt for the

Sahara's last switch from green

to desert, one that occurred in

the last 10,000 years.

To a geologist, opening this

core is like a portal back in

time.

>> Wow, that's amazing.

It's much, much redder in the

upper part of the core.

>> Laid down flat, every

quarter-inch

of sediment in the core

represents 200 years.

The color difference in the

sediment is subtle, but to a

practiced eye, it's a big clue.

>> When we split this core, what

is surprising about it is that

we see this really impressive

color change, and it goes from

this sort of darker green-brown

color in this section of the

core, which comes from the clay

minerals that make up the deep

sea sediments.

This bright red sediment

actually comes from the

windblown dust that's coming off

the Saharan desert.

As you move along this core, you

can see this color maintains

itself further up and up into

the core.

So, right now, we're about 7,000

or 8,000 years ago.

Boom!

Here is the drying of the

Sahara.

You can put your finger on it

in this core right here--

5,500 years ago.

>> Crucially, the proximity of

these two layers reveals how

quickly the switch happened.

>> The transition from a very

well-watered,

wet Sahara that was completely

vegetated to one that was much,

much dryer, that climate

transition in this core

occurred within one or two

centuries.

>> Scientists knew that the

Sahara was an ever-changing

environment.

Now for the first time, they had

a sense of just how fast it

changed.

As the Earth wobble shifted the

rain belt away, the return to

desert was swift and deadly.

>> These transitions would have

happened almost on a

generational time scale, that

one generation after the next

after the next would have

realized that where they're

living is no longer sustainable.

>> High above the ruins of the

lake settlement, Fekri Hassan

has discovered a cave he

believes was important to the

Saharan people during this

sudden change in climate.

Perhaps it holds eyewitness

clues to what happened.

>> When I first came into the

cave, the sand was as high as

this level, and I had to crawl

in because the sand had covered

the whole area.

Well, this windblown sand cannot

form when the desert is green.

>> Buried in the sand was the

first clue--some perfectly

preserved animal droppings.

>> So, these animal droppings

not only tell us about the

climate at the time, but they

also are excellent materials for

radiocarbon dating which allow

us to date the final event of

the drying of Sahara.

>> These goat droppings covered

in sand reveal a time when a

farming community was

overwhelmed by desert.

Hidden in the back of the cave

is a clue that confirms people

sought shelter here.

>> The cave has the very

interesting feature, which is

the prints of hands.

So, this is excellent evidence

of the people that lived here.

>> The next clue suggests that

around them, the Sahara was

beginning its relentless

transformation into desert.

>> Well, here we see a very

interesting drawing with these

long lines.

This represents a cloud with

rain coming down.

Rain was becoming very scarce at

that time, and they would have

used this cave to pray for rain.

>> But with the monsoon now

several thousand miles south,

their prayers could not be

answered.

>> This cave must have been a

very important sacred place for

these people at a time when

things were getting really bad.

>> Eventually, despite these

rituals, the force of this

change was so great, they had no

choice but to leave.

>> Here in this cave, we have

the sand, and we have the

handprints, which is the last

message left by the Sahara

population.

>> The story of this cave

started with a tilt in the

Earth's axis that stopped the

rain falling on the Sahara.

What must have seemed like a

never-ending

drought would, in just 200

years, turn a gentle, fertile

region the size of the United

States into a brutal, searing

wilderness, the wasteland we see

today.

This would be the biggest

environmental upheaval of the

last 10,000 years.

Those that could must have

migrated east to their closest

source of water--the valley of

the Nile, a beacon of green in

the vast desert.

This exodus had a surprising

outcome.

The death of one culture 5,500

years ago would lead to the

birth of one of the most

advanced civilizations on the

planet.

>> It was the drying of the

desert that led to this great

civilization.

People came from different

places in the desert,

established their villages, and

within a very short time, they

began to have the basic

ingredients for the rise of

Egyptian civilization.

So, climate change in this

particular case stimulated one

of the most spectacular events

in world history.

>> The investigation into how

quickly the Sahara returns to desert

has uncovered two striking clues.

Ostrich eggshell beads show

people and animals

inhabited a green Sahara

just 7,000 years ago.

Ocean sediments

show that the Sahara

returned to desert at breakneck pace,

In just 200 hundred years.

>> The next wobble

in the Earth's axis

is set for 15,000 years from now.

Only then, will the Sahara

turn lush and green again.

But now, modern technology

is finding ways to speed up

that process.

>> The investigation into the

Sahara desert's eventful past

now moves to the last 100 years.

In 1956, French prospectors

discovered vast reserves of oil

in the Sahara.

This triggered an oil rush that

led to drilling across the

desert.

Then they struck something

unexpected under the sand--huge

quantities of fresh water.

It seemed the Sahara had another

secret to reveal.

>> So, this is a classic pumped

well drilled for irrigating the

fields, and the water is

actually pretty hot, which

means it's coming up from a

considerable depth.

>> The deeper the well, the

hotter the water.

Water from wells in the Sahara

can reach up to 150 degrees

Ffahrenheit.

Such a high temperature means

the pump is drawing water from

far enough underground to be

warmed by the Earth's internal

heat.

>> So, pumps like these can

bring up water from 3/4 of a

mile underground or even deeper.

>> With no rain for years at a

time, this water must be coming

from somewhere.

Smith spots another clue.

>> Actually, when I look at the

surroundings of the well, I can

see some orangish red iron

staining.

>> This is hematite, a mineral

that is typically found in water

that's been underground for a

long time.

Combined with the temperature,

this points to some kind of deep

reservoir.

Scientists say that,

astonishingly, great quantities

of water lie under much of the

Sahara.

The key is in the sandstone.

Sandstone is made from layers of

sand compacted into rock over

millions of years.

>> So, what's really incredible

about sandstone like this is

just how good it is at holding

water, and that's because

there's a lot of pore spaces

between the sand grains that are

actually really big.

If I pour some water on this

rock, just like would have

happened when it rained over the

Sahara, it soaks right in.

>> Scientists now know that

every 20,000 years, a wobble in

the Earth's orbit shifts the

Monsoon north so rain pours

down onto the desert.

>> Much of that rain that fell

over the Sahara is now stored

underground.

>> It's called the Nubian

Sandstone aquifer, and like a

giant subterranean sponge, it

sits below Egypt, Libya, and

Sudan.

>> Though there is nothing but

sand and rock on the surface,

under the ground beneath my feet

is as much water as there is in

the Great Lakes.

>> The presence of a reservoir,

even one deep underground, is

surprising, given the Sahara's

searingly hot temperatures.

In 1922 in neighboring Libya,

the mercury touched 136 degrees,

a record still not beaten, but

ironically, the water is

protected by the desert itself.

Layers of clay encase the

sandstone.

The clay keeps out the harsh

sun.

It also acts as a sealant,

Trapping the water within the

rocks and creating pressure.

Fault lines in this clay are the

source of the desert's famous

Oases.

>> So, this is a natural spring

where water is coming up from

the Nubian Aquifer under its own

pressure.

This is actually fossil water.

It's been dated to be up to a

million years old.

>> This reserve of water is a

legacy of the Sahara's lush,

green past, the remains of its

giant lakes and rivers, and

this is just one aquifer.

Scientists are now using

ground-penetrating

radar to locate and map other

aquifers across the Sahara.

They hold the promise of even

more fresh water.

This new technology offers hope

that the desert may once more

turn green, reclaimed for

agriculture and farming.

If all goes to plan, eventually

there will be 200 wells here,

but drilling could prove a

short-term solution.

>> This is fossil ground water.

It's not being renewed, so

eventually you're going to run

out.

Water that supported

prehistoric occupants in the

area and accumulated over a

million years is potentially

going to be gone in less than a

hundred.

>> Once the underground water

dries up, the desert will have

to wait another 15,000 years

before, once more, the Earth's

wobble turns it green again.

The mystery of what created and

changed the Sahara desert has

revealed a turbulent past.

Whale bones in the desert show

that 40 million years ago, the

Sahara was a seabed.

Deep ocean cores containing

windblown sand reveal the date

it dried up--3 million

years ago.

Freshwater shells show that

90,000 years ago, a wobble in

the Earth's axis created giant

lakes and rivers and turned the

Sahara green every 20,000 years.

Ostrich eggshell beads indicate

that just 7,000 years ago, the

Sahara enjoyed its final burst

of life before returning to

desert.

The secrets of the Sahara have

finally been revealed.

This desert is not a static

wasteland.

It's dynamic and full of life,

capable of blossoming into lush,

green terrain.

This vast, majestic land

continues to shift, change, and

evolve,

much like the Earth itself.

-- Sync, corrected by elderman -- -- for www.MY-SUBS.com --