Inside Planet Earth Page #8

in a chemical reaction

which removed millions of tons

of the gas from the atmosphere.

With less greenhouse gas

to keep it warm,

the entire planet would cool,

triggering the ice age.

At the edge

of the vast ice sheets,

immense glaciers

advanced relentlessly.

Nothing could resist

as they raced forward at speeds

of up to 10 feet per day.

The ice covered more land,

so more of the sun's heat

was reflected back into space,

chilling the planet further.

At its coldest,

Earth was covered by 3 times

the volume of ice

on the planet today.

This ice locked the water away,

trapping it on the land.

With less available water,

the world's sea level dropped

by 425 feet.

Today the Caribbean island

of Grand Bahama

holds the clearest clues

to the dramatic change

in past sea levels.

Microbiologist Steffi Schwabe

has been piecing

the story together.

Hidden deep in a mosquito-ridden

mangrove swamp

is an unlikely entrance

to a mysterious ancient world.

These caves have a story to tell

concerning past sea-level fluctuation

and past interglacial and glacial periods.

What makes these caves

particularly exciting

is that they are one of the last

unexplored frontiers,

and they turned out to be

one of the world's largest

and longest known cave systems.

We move from the entrance into

a very large cathedral room,

where we can see the ceiling

comes up

to what we commonly call

these cave systems,

which is a blue hole.

You always find,

when you go into these

large cathedral rooms,

a very large rock pile

in the center.

And usually the collapse happens

when the water is no longer in

the cave supporting the ceiling.

I'm now swimming through

what is called a mixing zone.

This is where the freshwater

mixes with the seawater

and causes what we call

a shimmer.

This mixing zone

is responsible for aggressively

attacking the wall rock.

It's like soft cheese.

Normally, limestone could not

be removed with your finger.

You would need

a rock hammer and a chisel.

This is how we know

that the caves have formed

hundreds of thousands

of years ago.

But we also know

by evidence that we have found

that the caves have been dry

in the past.

20 meters down further

into the cave system

are these bat droppings

that have been fossilized.

And we know the only way

that this can happen

is when the caves are dry,

because bats do not swim.

And when bats roost

in the ceiling,

the droppings collect

on the rock,

and they become very hard.

Deeper into the cave system,

we find another bit of evidence,

and that is

this red Saharan dust,

which gets blown

into the stratosphere

during frequent storms which

occur in the Sahara Desert

and will actually find its way

into these caves.

Most likely it's happened during

periods when the cave was dry.

The final bit of evidence

which supports the fact

that the caves have been dry

are these beautiful stalactite

and stalagmite gardens

that we find at 28 meters-plus.

Stalactites and stalagmites form

during the ice ages

and have formed over a period of

hundreds and thousands of years.

What happens is

the water is frozen

in the ice caps on the poles,

sea level drops,

and the caves become dry.

And usually during ice ages

it rains a great deal more

in the tropical regions

of the world.

This rainwater percolates

through the bedrock,

or the ceiling rock,

and dissolves the rock

that's there,

and it comes out in the

drip water and crystallizes,

just like an icicle.

And basically

these are rock icicles.

The ice ages affected and

shaped the landscapes.

But sometimes the clues

are so gigantic,

it's hard to recognize them.

In Washington state

in the Grand Coulee Canyon

are the channeled scablands.

For years, a succession

of scientists tried to work out

what could have caused erosion

on this scale.

USGS scientist Richard Waitt

is the latest investigator.

This rock, the size of

a 3-story house,

looks like it's part of the bedrock,

part of this basalt

that forms this vast landscape.

But the structure in the basalt

is horizontal, as you can see,

whereas the structure

in this rock is vertical.

In other words,

this big rock has been moved.

But the biggest clue

in the landscape

is the landscape itself.

This gorge is carved in basalt,

one of the hardest of rocks.

It ends in a sheer cliff

400 feet high,

and it's in the shape

of an enormous horseshoe.

It's like the gorge below

Niagara Falls.

But this, Dry Falls,

is fully 2 Niagaras high and 3 wide.

Waitt is continuing the work

begun by geologist

J. Harlen Bretz in 1923.

Bretz was the first to say

that this entire landscape

was a gigantic riverbed

formed by 3,000 square miles

of the Columbia Plateau

being swept away.

No one believed him

because the scale was so huge.

But he was certain

that the only explanation

for this scale of damage

was an enormous flood.

The whole are was obliterated

and changed forever

as the torrent flooded through.

Even on the top of the cliffs,

the water

was over 100 feet deep.

The basalt cliffs

are amongst the hardest

structures found in nature.

Resistant to weathering

and erosion,

these walls

could only have been cut

by an enormous body of water.

One by one,

these columns of frozen lava

were quarried from the rock face

and carried away

by the raging torrent,

causing the wall to retreat

and the cataract to widen.

So it was accepted

that this is how the

channeled scablands were shaped.

But an important part

of the mystery remained--

just where could all that

colossal, earth-shattering

volume of water have come from?

100 miles to the east,

geologist Joseph T. Pardee

had described

an enormous Ice Age lake--

Glacial Lake Missoula.

This lake held in some

600 cubic miles of water.

600 cubic miles

is all of present-day Lake Erie

plus all Lake Ontario.

The lake was held in

by an ice dam

that had crossed the

Clark Fork River and dammed it.

The lake rose 2,000 feet deep

against the side of the ice dam.

But there was no evidence

to link the lake

with the channeled scabland.

In 1939, Pardee discovered

a series of giant ripples.

These are like sand ripples

on the beach,

but they are of enormous size.

Hundreds of feet apart.

10 feet high and more.

And they're composed of gravel.

Such a feature could only

indicate a swift outflow

from Glacial Lake Missoula.

The gigantic ice dam

held back the water

until it reached a critical level.

When it was 1,800 feet deep,

the pressure of the water

was so immense

that it forced its way

through the base of the ice dam.

Having found a weakness,

the icy waters raced on,

widening the split

and weakening the dam

catastrophically.

The waters of the entire lake

were released

in a devastating discharge.

This discharge was

10 times the flow

of all the world's rivers.

That's almost beyond belief.

In 1926, a freak flood created

a 100-foot-deep canyon

200 miles away

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