All language subtitles for Inside.Planet.Earth.2009

NARRATOR: What do we really know
about the planet we live on?

This giant spinning ball
of rock.

The truth is,

something extraordinary is
going on deep inside the Earth.

Powerful forces,
mysterious processes

are happening thousands of miles
beneath our feet.

And without them, life on
our planet would be impossible.

The secret to life on Earth
lies inside.

[Rumbling]

To discover how and why, we need
to crack the Earth open...

...and travel all the way
to the core.

A century ago,

Jules Verne's book "Journey
to the Center of the Earth"

captured the world's
imagination.

Of course, in reality,
it's an impossible journey.

In the center of the Earth,

there are titanic pressures
and extreme temperatures.

They make 99%
of the planet beneath us

inaccessible to humans.

It is easier to design something
to descend into the sun

than it is to design something

to go to the center
of the Earth,

because the temperatures
are as high or higher

than the surface of the sun,

but the pressures
are unimaginably large.

NARRATOR: Because scientists
can't travel to the core

and see for themselves,

they have to work out other ways
to understand it.

It's not easy studying something

you'll never be able to see
or touch.

LATHROP:
We can see hurricanes coming.

We can see fronts coming that
will have violent thunderstorms.

All of that predictive power

comes because
we can observe the atmosphere.

We don't have anything like that
in the interior of the Earth

because we don't have
any detailed measurements

of what's happening in the core.

We don't really know
any of the motions in the core.

We don't know how
the temperatures are varying.

We don't know what storms
are brewing down there.

NARRATOR: But Lathrop
is determined to find out,

so he's building
his very own planet Earth

at the University of Maryland.

So we've been seven years in
construction of this experiment.

Built to try to match
as many parameters as possible

with the Earth's core.

It's a model of both the outer
and inner cores of the Earth.

NARRATOR: It might look like
a crazy experiment,

but investigating
the Earth's interior

is more than just
scientific curiosity.

Life on Earth's surface,
where we live,

actually depends
on processes taking place

deep inside our planet.

If we can figure them out,

then we'll be closer
to understanding

how and why life exists
and what its future could be.

LATHROP:
And the hope is,

by building a laboratory model
of a planetary core,

or the Earth's core,

that we can probe in detail
what's happening

and work toward getting
a predictive science,

being able to predict
what's going to happen

toward the future
for the Earth's core.

NARRATOR:
Lathrop is not alone.

Around the world, scientists are
probing the planet

in every way possible

to solve the mysteries
of the deep Earth.

They're studying volcanoes...

...measuring vibrations
from earthquakes

to perform seismic X-rays
of the planet...

...building complex
laboratory models...

...and discovering
that the world beneath our feet

is stranger and more fantastic
than they could ever imagine.

It's full of incredible riches,

monumental structures,
and bizarre creatures.

They've found there's actually
more life beneath the surface

than above it...

...and more water
than in all of the oceans.

Down here,
there are even raging storms

more violent than
the planet's worst hurricanes.

And somehow this mysterious
world deep inside the planet

shapes our own.

But to discover how
is a huge challenge.

LATHROP: Almost any basic
quantity that you imagine

might be changing down there.

There's a whole host
of interesting questions

that you'd like to know
about the core

but that you can't
unless you go there.

NARRATOR: There are many
mysteries in the deep core

but perhaps none so powerful
as gravity.

Gravity keeps the moon and
thousands of man-made satellites

in their orbits.

And even out here

it prevents molecules of gas
from floating off into space.

This immense force

comes from the massive
dense interior of our planet.

The closer we get to Earth,
the stronger this force becomes.

By 62 miles up,
gravity has collected enough gas

to form a cocoon
around the Earth.

This is the Earth's atmosphere.

It protects us from meteorites,
absorbs lethal radiation,

and insulates the Earth from the
freezing temperatures of space.

And what's most important...

It gives us the air
that we breathe.

It's simple.
No gravity... no atmosphere.

No atmosphere... no life.

There's another force of nature
inside Earth

that's just as vital to life.

We take it for granted that life
gets its energy from the sun.

True, its nuclear furnace
does warm our atmosphere,

drive our weather,
and make our food grow.

Without the sun, life on Earth
would quickly disappear.

But forces
from deep inside the Earth

played a vital role in creating
life in the first place.

Life survives today
because of a careful balance

between the energy of the sun
on the outside

and the energy coming
from inside Earth's core.

The most visible sign

of the seething energy
inside our planet are volcanoes.

They erupt through cracks
in the crust,

the planet's
fragile outer shell.

This layer
is only 30 miles thick.

All of the Earth's volcanoes

release just a tiny fraction
of the energy

locked beneath the surface.

The Earth's inner energy
is so powerful,

it can thrust rock layers
high in the air,

creating whole mountain ranges

such as the Guadalupe Mountains
in New Mexico.

These layers
were once a flat seabed

until the Earth's heat pushed
them 8,000 feet into the sky.

In this churning,
heaving action,

cracks and fissures
let in water,

which dissolves
the soft limestone rocks

below the surface.

Here in New Mexico are the
magnificent Carlsbad Caverns.

One chamber is so large,

it could comfortably accommodate
a jumbo jet.

For Peter Scholle,

these caverns are
a geological treasure trove.

We're 850 feet below
the surface of the Earth here

in the lower cave
of Carlsbad Caverns.

We are amongst a bunch

of limestone stalactites
and stalagmites.

This cave has probably
a couple of miles of passage.

There are other caves

that have literally
hundreds of miles of passage.

In many cases,
there are actually rivers

that flow through them for tens
or even hundreds of miles.

NARRATOR: The eerie stalactites
growing downward

and the stalagmites
growing upward

were deposited by the water
over thousands of years.

[Rumbling]

Our journey
from the surface to the core

reveals more
spectacular surprises

as we head further downward.

Just below the surface,
it's cold, dark, seemingly dead.

Then, very quickly,
everything changes.

As we go even deeper,
it gets warmer, then hot.

The next stop on our journey...

a mysterious cave
below the Mexican desert.

This is what the Earth's
inner energy can do.

At nearly 40 feet long,

these are the largest known
crystals in the world.

They're what's left
of an underground lake

rich in minerals.

The lake was turned into
a boiling cauldron

by red-hot magma
erupting from below.

As the hot water
percolated through the crust,

these giant crystals

grew from the minerals
dissolved in the water.

Today, the chamber is still
a scorching 120 degrees...

so hot, scientists can only work
30 minutes at a time,

even in their
climate-controlled suits.

LATHROP: But the deep interior
is quite unsuitable for people.

Pressures are high,
temperatures are high.

And early on,
people going to mines

realize it gets hotter
as you go deeper.

And so there's
this fascination then

with this inhospitable interior

to what is otherwise
a pleasant surface we live on.

NARRATOR: But the energy
inside the Earth

can do more than make mountains
and hollow out caves.

In the 1960s,

scientists discovered
it can move entire continents.

The Earth's crust is formed

from seven massive sections
called plates.

What researchers realized

is that these plates
were all shifting.

In some places,
they're pulling apart,

in others, smashing together.

Mountains are the crumple zones
of these collisions,

and some are truly spectacular.

These are the Swiss Alps,

where two continents
crashed together.

High peaks, like the Matterhorn,

testify to the immense scale
of the forces unleashed.

It's literally a piece of Africa
sitting on top of Europe.

Every year, these mountains
grow by a quarter inch.

The Earth is always in motion.

Our mountains and continents
slide around the Earth's surface

driven by energy
from deep inside the planet.

But as this driving force
reshapes the surface,

it reshapes life as well.

It can change and transform
the course of life.

The evidence is here...

1.5 miles down inside
a vast coal seam.

700 miles long
and 120 miles wide.

212 million tons of coal.

All the coal on Earth

is the fossilized remains
of a superforest

that once dominated
the surface of our planet.

360 million years ago, there was
an explosion of life on Earth.

It was more diverse,

more abundant
than it's ever been since.

And it was all because
of the way

that forces inside planet Earth
had shaped the surface.

Go back in time.

That driving energy
at the heart of the planet

had pushed
the continents together

into a single giant landmass
wrapped around the equator.

On this supercontinent,
known as Pangaea,

there were vast lowland swamps
and tropical rainforests.

It was a massive hothouse

and led to the creation
of millions of new species.

This period of time is known
as the Carboniferous era.

The closest scientists can get

to those conditions on Earth
millions of years ago is here...

the Okefenokee nature reserve
in southern Georgia.

Dr. Fred Rich is exploring
how the inner Earth and life

are interconnected.

DR. RICH: There were
large landmasses at the equator.

So you have to imagine this flat
landscape just above sea level,

very well-watered,
in the tropics.

And that paleogeography
and the weather conditions,

the meteorology
that followed from that,

led to the appearance of forests

that were unlike anything that
had ever existed on the planet.

NARRATOR: It wasn't just
that the forests were big.

The trees were monsters, too.

DR. RICH:
Huge plants...

Some of these are reckoned
to have been 70 to 100 feet high

and perhaps as much
as 5, 6 feet in diameter...

...lived across
this immense moist landscape.

And plants grew
until they got so big or so old

that they simply fell over.

NARRATOR: These huge trees
and dense forests

had a profound effect
on the atmosphere.

They sucked up carbon dioxide
and pumped out oxygen.

DR. RICH:
High humidity.

Tremendous amount
of oxygen exchange.

I mean, these plants
were photosynthesizing.

So, understandably,
these were oxygen pumps.

And they were similarly
pulling huge amounts of CO2

out of the air.

NARRATOR:
360 million years ago,

the proportion of oxygen
in the air

was 60% greater
than it is today.

The high levels of oxygen

led to another dramatic effect
on the Earth's creatures.

It supersized them.

There were poisonous centipedes
6 feet long.

2-foot cockroaches.

Even dragonflies
the size of sea gulls.

DR. RICH: Dragonflies that we
find in this swamp are large,

and they're certainly numerous.

But the dragonflies
of the Carboniferous

would have been much bigger.

Easily three, four times
the size,

based on what we have
for fossil evidence.

NARRATOR:
Instead of alligators,

the dominant predators
were giant toads.

Alligators would have been
replaced by large amphibians...

amphibians probably as large
as the alligators

that we have
in these modern swamps

but looking differently,
perhaps.

NARRATOR: New species that
changed the evolution of life,

all because the energy inside
our planet reshaped its surface.

This strange lost world
existed long before humans,

but its story was sealed
into the Earth's rocks in coal.

The forest first became peat.

This was then squeezed
under tons of rock,

where it started to dry out.

Now, in the process
of this brown messy sediment

becoming coal,

the first thing we would need
to do is get rid of the water.

Earthly processes do that simply
by loading the sediment.

So the longer the sediment
is in the ground,

the longer it has been buried,
subjected to geothermal heat

that's coming from
the interior of the Earth,

the more the sediment
is compacted,

and the more the water
is driven out.

NARRATOR:
So the Earth's internal energy

had reshaped the landmass
to make life possible,

then broke it apart and buried
the remains deeper and deeper

until the heat and pressure

transformed the ancient forests
into coal...

fossilized remains
of a lost era.

As we go deeper on our journey,

there are other riches
for humans to exploit.

2.5 miles down,

we pass a glittering seam
of gold being formed.

Boiling fluids
full of dissolved gold

bubble up
through the cracks in the rock.

The higher it rises,
the cooler it gets,

until the dissolved gold
finally settles into seams.

Earth's thin crust...

home to life in all its complex,
colorful, infinite variety.

Below it is an inhospitable,
lifeless world.

Or so it seemed.

Scientists are now finding life
deep inside Earth.

It's a remarkable discovery made
in the world's deepest mines.

This is the Witwatersrand region
of South Africa.

The mines here reach
2.5 miles inside Earth's crust.

It seems like they stretch
a long way down.

But in reality,
they barely scratch the surface.

This is a hostile environment
for a human being.

It's 130 degrees Fahrenheit,
100% humidity,

and extremely cramped.

The mines are so deep,

the miners have to descend
in two stages.

A single elevator cable
stretching 2.5 miles

would snap under the strain.

It's so far down,
the journey can take two hours.

Like the miners,

these biologists
from Bloemfontein University

risk heatstroke as they descend
into one of the mines.

But they're not interested
in gold.

They're looking for life...

colonies of
extraordinary creatures

that thrive
in these extreme conditions...

...bacteria they believe
may be direct descendants

of the very first
life-forms on Earth.

Leading the team
is Professor Derek Litthauer.

You've got
communities of bacteria.

And possibly even fungi.

We don't know yet.

But probably mostly bacteria
living in there.

And the kind of populations
you get in there

are usually determined

by the chemical composition
of the water.

But our past experience

has been that there's
some unique stuff in there.

NARRATOR: The scientists tap
into ancient underground water

released during
the mining process.

The water
and the bacteria inside it

have remained undisturbed
for billions of years.

These bacteria are tough.

All they need to survive is
rock, water, and scorching heat.

LITTHAUER: There's an amazing
diversity of life underground,

even in the deep subsurface.

In some areas,
we can expect life

possibly even down to
10 kilometers below surface.

And they are
extremely sophisticated,

very highly specialized for the
environment in which they live

off the nutrients
that they can get in the rocks.

NARRATOR:
It's an extraordinary discovery

that has transformed
biologists' understanding

of the origins of life.

The bacteria
are the latest additions

to a strange group of creatures

that thrive
in extreme conditions

called extremophiles.

[Bubbling]

In the 1960s,

astonished scientists
found bacteria

living in Yellowstone's
boiling acid pools.

Then in the 1970s,

biologists discovered life
1.5 miles down in the oceans

close to vents in the seafloor
called black smokers.

These life-forms thrive
on nothing more

than volcanic gases.

If life exists
in such hostile conditions...

...it suggests
a teeming mass of life

could exist beneath our feet.

It's been estimated that all
the bacteria inside Earth

could weigh more than all the
life aboveground put together.

It also raises
an intriguing possibility...

that life may have started
not on the surface

but deep within the Earth.

There's more diversity and more
life in the deep subsurface

than we have above surface.

The implications for this,

in terms of the evolution
of life in the universe,

are quite astounding,

because the old concept
that life could have started

in very calm, serene, warm pools
on the surface of the Earth...

That may be completely wrong.

Life may have started
in the subsurface.

NARRATOR:
If life began underground,

then somehow at some time
in Earth's history,

it found a route to the surface.

Perhaps the Earth's
inner energy,

as it pushed through the crust,

took the extremophiles
to the top.

Or maybe it hitched a ride
on a black smoker...

a kind of extremophile elevator
to ground level.

Or floated up
in thermal hot springs,

boiling up
from deep in the Earth.

How far down primitive life
could survive is uncertain.

But to explore what lies
beyond the deepest mine

pushes technology to its limits.

The only way down this far
is to drill.

But pressure and heat
put a limit

on even our most
sophisticated drill bits.

The deepest hole ever drilled
bored just 7.5 miles

into the Earth's 30-mile crust.

In the 1970s,
the Soviets race

to drill the world's
deepest borehole in Russia.

The drill bit was so long,

it bent and stretched
like a piece of elastic.

But even at this depth,

we are less than halfway through
the Earth's surface layer,

the crust.

It's only 1/500th
of our journey to the core.

7.5 miles is like traveling
from downtown Chicago

into the suburbs.

But it's another 4,000 miles
to the center of the Earth.

That's like commuting
from Chicago to London.

Scientists may be restricted

to exploring the thin top layer
of the Earth's crust,

but their journey of discovery
isn't over.

They've found other ingenious
ways of exploring inside Earth,

and in the process

discovered more
surprising connections

with the evolution
of life itself.

Life on Earth might have started
deep in the Earth's crust,

but violent upheavals
even further down

played a crucial role
in pushing life

on to the next stage
of evolution...

one that would lead to all life
as we know it.

Remarkably, we know this
from the ancient rocks

of the crust itself.

These mountains

in Western Australia's
Karijini National Park

are made from rock
that's 3.5 billion years old.

They used to be the bed
of an ancient sea.

Their red color
comes from iron ore

imbedded right in the rock.

But the iron is evidence
of something remarkable...

...because they were formed

during one of
the most important events

in the story of life on Earth.

The bands of red iron ore
were once layers of sediments,

and they contain evidence
of very primitive life-forms.

Martin Van Kranendonk
is a geologist

who's spent a lifetime
studying these rocks.

Each one of these little bands

is only about
the length of a thumbnail,

and it was maybe deposited
in a year.

So you can see here,

you've got hundreds of feet
of deposited sediments.

It represents
hundreds of thousands of years

of geological time.

NARRATOR: These were
no ordinary sediments.

They contain fossils of rock
structures called stromatolites,

created by some
of the earliest living things...

simple bacteria.

It's hard to imagine,

but this immense volume
of iron-rich rocks

was actually formed
by tiny microscopic organisms

that formed structures
such as preserved here

in this very old rock.

This is an example
of a stromatolite

that's built by single-celled
organisms in this rock,

which is 3.45 billion years old.

This is the oldest fossil
on the planet.

NARRATOR:
Incredibly, these bacteria

are still making these
distinctive rock formations

just 400 miles to the west.

These strange-looking mounds
are giant stromatolites

built by the bacteria.

VAN KRANENDONK:
Well, stromatolites are rocks,

but they're rocks that are made
by living microorganisms

or, as we call them, microbes.

And so these stromatolites
actually grow

by precipitating rock.

So they build up layer by layer,
but only very slowly.

NARRATOR: The bacteria
also produced something else,

something which kick-started
a biological revolution...

oxygen.

If life did start underground,

maybe it eventually
found its way to the surface,

propelled upward
by those forces within Earth.

And once
they'd reached the surface,

those bacteria found a new way
to harness energy,

not from the rocks
and the heat of the deep Earth,

but from sunlight... the process
we call photosynthesis.

And one of the most important
by-products of photosynthesis

is oxygen.

These stromatolites
are incredibly important for us.

They're really the precursors

to allow life to evolve
from the oceans on to land

and to breathe air.

NARRATOR:
Without oxygen,

complex life as we know it
simply wouldn't exist.

But oxygen also changed
the composition of the planet,

creating the iron ore
in the crust.

At the time,
most of the iron on the surface

was dissolved in the oceans,
making them appear bright green.

But the newly released oxygen
bonded with all the iron

to make iron oxide, or rust.

The iron oxide
fell to the seafloor,

and the seas turned blue.

Eventually, the iron oxide
formed the deposits

we see
in the Karijini mountains.

Layer upon layer of iron oxide
exists in the Earth's crust

thanks to primitive bacteria.

It's the ore
from which we extract

1.7 billion tons of iron
each year,

and it's also rich in oxygen.

In fact,
there's 20 times more oxygen

locked up in the bands
of iron ore

than there is
floating in the atmosphere.

It's another example
of how the world we know

has been shaped
by the incredible forces

deep inside the planet.

But where do
these forces come from?

We now enter
the part of the Earth

that holds the answer...
the mantle.

It's a dynamic mass
of churning rock

kept moving by energy
from the core...

the powerhouse of the planet.

Below the 30 miles
of surface crust,

we now move deeper, further
than any human has ventured,

into the Earth's mantle.

The mantle is the real key

to understanding
how our world works.

When you see flowing lava,

it's easy to think
that the mantle is liquid.

In fact, it's nearly
2,000 miles straight down

of hot but solid rock.

It makes up 80%
of the Earth's volume.

Nothing can live here.

But what happens at these depths
is vital to life on Earth.

The mantle
may be beyond our reach,

but sometimes it reaches us.

The solid rock liquefies

when the massive pressure on
the mantle is suddenly released

through fissures and cracks
in the crust.

The radical change in pressure
transforms the rock into lava.

The rock of the mantle
beneath the Earth's crust

is inaccessible.

But against the odds,

there are some places
where mantlerock

has been forced to the surface.

One of them
is on the Lizard Peninsula

on the southernmost tip
of England.

On this peaceful beach

is evidence of something violent
and powerful...

a piece of mantlerock
that broke away

and was forced upward 30 miles

by the churning movements
of the crust.

For geologists like Robin Shail,

it's the perfect place
to study mantlerocks,

which are normally
way beyond his reach.

How do they compare
with other rocks on the surface?

What do they tell us about
what's inside planet Earth?

DR. SHAIL: The rocks here
look completely different.

They have colors which vary
from greens

through to oranges and yellows.

This is typical of mantlerocks

wherever they're exposed
at the Earth's surface.

NARRATOR:
Like no other rocks we know,

mantlerock is very hard
and very heavy,

nearly twice the weight
of granite.

It's a dense mass of minerals
rich in heavy elements

such as iron and magnesium.

And it's the source of gemstones

such as the distinctive
green peridot.

Close up,
structures are revealed

that could only have been formed

under extreme temperature
and pressure.

Here on the Earth's surface,
this rock seems solid enough.

Deep underground, however,
it becomes very different,

something that behaves
more like fudge.

When we look at this mantle
peridotite, it appears solid.

In contrast, when mantlerocks...
or fudge... are warmer,

you can actually stretch
and make it flow.

And the significance for this

is that these weak layers
within the mantle

allow the overlying plates
to move slowly across.

NARRATOR: A solid that flows
may seem strange,

but the mobility of the mantle
is vital to life on Earth.

Because currents of heat
circulate upwards

from the core
through the mantle,

the plates of the crust
can move around on the surface.

Without this shifting geology,
there'd be no continents,

and the conditions for life
would never have existed.

DR. SHAIL: Without these zones
in the mantle

that allow the plates to move
across the Earth's surface,

we would basically have
a geologically dead planet.

We would have no plate movement.

We would have
no mountain ranges.

We would have
no major ocean basins.

So the mantle
is absolutely critical.

NARRATOR: These are
the deepest rocks visible

on the Earth's surface.

To look further into the mantle,

scientists must find
another way.

Inside the Earth's mantle,

crushed beneath
100 miles of rock,

the pressure
is 50,000 times more

than we feel at the surface,

like carrying 20 Titanics
on your shoulders.

It's in this hostile environment

that some of the Earth's
greatest treasures are forged.

The pressure creates diamonds.

It crushes carbon
into the hardest mineral

known to science.

But we don't have to dig
100 miles to find them.

Diamonds exist just a few
hundred feet below the surface.

They were forced up
through the crust

by violent prehistoric eruptions

triggered by
the Earth's internal heat.

Today, miners excavate
these extinct volcanic vents

in search of diamonds.

The Letseng diamond mine

is located in the mountain
kingdom of Lesotho...

...a small country
in the heart of South Africa.

The diamonds are imbedded
in rock called kimberlite

inside an old volcanic pipe.

It's the job
of company geologists

like Claire Palmer
to find them.

DR. PALMER:
We're standing within the pipe,

the original
eruptive pipe that formed.

And the original earth surface
would have been

at least 200 meters
above our heads.

And we're actually,
in the mining process,

reexcavating that pipe.

NARRATOR:
Most of the diamonds on Earth

exploded through the surface
during huge volcanic eruptions

one billion years ago.

DR. PALMER: These volcanoes
erupted at supersonic speeds.

So you can imagine the power
with which it explodes.

Similar to that
of Mount St. Helens.

But Mount St. Helens' eruption

moved laterally
across the Earth,

whereas these eruptions were
actually a lot more vertical

in their expanse.

NARRATOR:
These violent eruptions

exploded minerals
from 100 miles down

upward to the surface
in minutes.

Today, the diamonds are locked
inside this volcanic rock.

There's only one way
to get them out.

Letseng is a valuable mine.

All these diamonds
were recovered

in just over two weeks.

These diamonds
are known worldwide

for their very high quality

and yield the highest dollar
per carat in the world.

The Letseng diamond mine

is famous for its
very large diamonds.

One of our most famous
is the Lesotho Promise...

603 carats, which was recovered
in August 2006.

And it sold on tender
for $12.4 million U.S.

NARRATOR:
Not all diamonds are perfect.

Some have microscopic flaws.

A perfect diamond
is worth a lot more money.

But for geologists, these flaws
are the real treasures.

They're tiny fragments
of primitive mantle

trapped inside the diamond,

and they're the deepest samples
it's possible to capture.

They tell a remarkable story.

Like time capsules,

they hold the key
to unlock secrets

of the Earth's
very early history.

From their chemistry,

scientists can deduce
that most of these diamonds

are 3.2 billion years old.

They can even figure out
they were forged 100 miles down.

Diamond samples from
different parts of the world

show large variation
in their composition.

That suggests the mantle
was a churning dynamic place,

even in the early history
of the planet.

From below 100 miles,

very few rock samples
reach us on the surface.

But this isn't the end
of our journey to the core.

There is another way
to see what's down there.

It's like an X-ray image
of planet Earth.

Most of the time,

we're unaware of the power
locked inside our planet.

But sometimes
there are violent reminders.

[Rumbling]

Earthquakes are the result
of processes

taking place
deep in the interior.

Propelled by the slow movement
of the mantle,

the great plates
that make up the Earth's crust

constantly grind into, over,
and under each other.

Pressure builds
until something snaps.

When this happens, the Earth
shakes, heaves, and rolls.

The results can be catastrophic,

especially when they happen
in populated areas.

This earthquake in China in 2008

killed 70,000 people and cost
$150 billion worth of damage.

Big earthquakes are disasters,

but they're also windows on
the deep interior of the planet.

Scientists can make use of the
shattering power of earthquakes

to help understand
the Earth's most remote depths.

They use a worldwide network
of devices called seismometers

to trace earthquake vibrations

as they travel
through the planet.

The data produced
can help fill in our picture

of the deep Earth.

Professor Ed Garnero uses this
technique to study the mantle...

all 1,800 miles of it.

GARNERO:
When an earthquake happens,

the waves travel away
from the earthquake

through the planet in the
interior and on the surface...

in the same way, when you drop
a rock in a pond,

you see the rings getting
bigger and bigger and bigger

from the drop zone.

So, what we do in seismology is,

we have these sensitive
microphones all over the planet

that record the ground shaking.

And so we keep track of
the precise time it gets here.

So when you use a bunch
of these instruments in concert,

you can start to say something

about the material
the waves travel through.

NARRATOR:
Just as doctors use sound waves

to picture a baby in the womb,

the waves from earthquakes
can tell scientists

about the world concealed
deep beneath the Earth's crust.

The waves travel through
and bounce off structures

within the planet.

GARNERO: So if you have
enough seismic data,

you can start to characterize
the shapes of things

inside the planet that are
reflecting the seismic energy.

NARRATOR:
And because earthquake waves

travel differently
through different materials,

we know our planet
is made of many layers,

like an onion.

The waves show the mantle
extends downward for 1,800 miles

and offer the first glimpse
of our ultimate destination...

the Earth's core.

Ed Garnero's results

show intense activity
within the mantle.

They reveal how convection
currents of hot solid rock

constantly circulate
through the whole layer.

It's too slow
to observe directly.

But speed it up
and it's clear...

over millions of years...
the mantle is in constant flux.

Resembling mushrooms,

the vertical columns
in his animations

show the steady movements
of the Earth's interior.

GARNERO:
So, what we're looking at here

is a convection calculation
depicting things...

When they get to the top,
they cool off, and fall back in.

Just like a lava lamp,
you know,

the blob goes up
and then its heat goes away

and it falls back in.

So that's
what's happening here...

the cycling of material
in Earth's mantle

over millions of years.

And this is a process
that's happening today.

NARRATOR: These convection
currents through the mantle

transfer heat from the core
to the crust...

heat that drives and pushes
the continental plates

on Earth's surface.

In this way,

the roaring energy of the core
shapes the world we live in.

The crust consists
of two kinds of plates...

oceanic plates
and continental plates.

Ocean plates are heavier,
so when the two collide,

the oceanic plate
plunges downwards

under the lighter
continental plate.

Whole sheets of crustal plate

extend right down
to the edge of the core.

GARNERO:
As that plate descends

and drags some of the water
down with it

and the water...
some of the crust sediments

are still saturated...
they make their way down.

That water can actually
be stored in the mantlerock.

NARRATOR: Over millions of
years, descending ocean plates

have dragged
so much water into the mantle

that scientists estimate
there's now more water

below the Earth's surface
than above it.

GARNERO:
Take all the water

from the oceans and lakes
and glaciers...

everything on the surface
of the Earth...

and anywhere between
2 and 10 or 12 amounts of that

can actually be stored
in the Earth.

NARRATOR: If all this water
rose to the surface,

there would be flooding
on a biblical scale.

No land could survive.

Eventually,
sea levels would rise

21/2 miles above
the peak of Mount Everest.

Luckily for us,
it will never happen.

But some of this
underground water

does make its way
back to the surface.

The water carried down
by ocean plates into the mantle

become superheated and drives
back toward the surface.

A change in pressure liquefies
the hot mantlerock.

Mixed with expanding water,

the lava punches up
through the crust,

where it erupts
with spectacular force.

Mount St. Helens is
the most famous American volcano

created at a plate boundary.

The pulverized rock and steam
that billowed out of the volcano

following its 1980 eruption

was once part of the plate
beneath the Pacific Ocean.

There's a ring of explosive
volcanoes like Mount St. Helens

circling the Pacific Ocean.

It's called the Ring of Fire.

Each one marks the spot

where the Pacific plate
dives into the mantle below.

We're now entering
the lower mantle,

a region at the edge
of scientific understanding.

Nobody knows what it looks like,

but scientists speculate
the hostile conditions here

may create
bizarre chemical effects.

GARNERO: If you were to be able
to go into the mantle,

you would see exotic things,

chemical things
that we're not quite

we fully understand right now,
but there's evidence for it.

And you'd see a lot of different
kinds of layering.

Just like when
you're driving in your car

and you see a roadcut,
you can see the layered rock.

NARRATOR: But in a few places,
something disturbs these layers.

Plumes of hot mantlerock rise up
from the core to the crust.

If you happen to live above
one of these plumes,

the result can be both creative
and destructive.

So you would see
little isolated conduits...

the details of which
we're not fully clear on,

but we think they could be
100 miles in diameter...

very hot material that works
its way to the surface

and gives rise to these things
that we call hot spot volcanoes.

You can see in this image,

you have hot plumes of material
coming up to the surface.

And the stuff that comes out
is what we see coming out

of places like Hawaii
and Easter Island

and Kerguelen Islands and such.

And this animation was made
with things called tracers...

these little black dots.

So you can get an appreciation
for how slowly the material

moves across
the core-mantle boundary

until it finds its little plume
upwelling and then... foom...

they shoot up quite rapidly.

NARRATOR: Some of the world's
largest volcanoes...

Yellowstone...

Iceland...

Hawaii...

sit right above
these gigantic mantle plumes.

Hawaii's Big Island is evidence
of their creative power.

Measured from the ocean floor,

this is the world's
tallest single mountain...

4,000 feet higher
than Mount Everest.

And every foot of it
is made from lava

spewed out from the top
of a mantle plume.

The surface plate
is constantly moving,

while the mantle plume
stays still,

so the magma
keeps punching through the crust

in different places

and leaves a chain of extinct
volcanic islands in its wake.

But while mantle plumes
have the power

to create entire island chains,

they also have the power
to destroy vast amounts of land.

Yellowstone's geysers and
mud pools may delight tourists,

but they are signs
that the park sits on top

of a vast mantle plume.

With a crater 45 miles long
and 35 miles wide,

this is one of the world's
largest supervolcanoes.

Geologist Hank Heasler wants to
understand its behavior.

DR. HEASLER: There's been many
destructive volcanic episodes

in Yellowstone...
three massive eruptions...

one at 2. 1 million years ago,

which is one of the largest
that we as geologists can define

on the face of the Earth,

one at 1.3 million years ago,
and one at 640,000 years ago.

NARRATOR: Yellowstone may not
look much like a volcano.

It's more of a wide depression.

But that's just because
of its sheer size.

DR. HEASLER: Yellowstone
is such a big volcano

that so much material
has been erupted...

hundreds to thousands
of cubic kilometers of magma

have been forcefully ejected
into the air.

When all that magma is erupting,

the ground actually subsides
into the void

created by the erupting magma.

NARRATOR:
It's been 640,000 years

since Yellowstone last erupted.

Heat emissions from the park
could be a sign

that the next eruption
is overdue.

If the Yellowstone volcano
does erupt,

it will unleash billions of tons
of ash and gas

into our atmosphere.

It would block out the sun
and plunge the world

into a devastating
volcanic winter.

Mantle plumes are a key part

of the Earth's
interior cooling system.

They have the power to create

some of the world's
most beautiful

and dangerous landscapes.

The question is,
what creates mantle plumes?

Nobody knows for sure.

But one thing is certain...

The answer lies somewhere
in the boiling furnace

of the Earth's core.

1,800 miles
down into the Earth,

just below us, is the core.

The Earth's outer core
is a huge ball of liquid metal

bigger than the moon.

LATHROP:
The conditions of the outer core

are really quite hostile.

Temperatures more
than 3,000 degrees.

The pressure
is just mind-boggling.

More than a million atmospheres
of pressure.

If you could
strip away the mantle

and just have the raw core,

it's quite hot
and would be glowing intensely,

very much like the surface
of the sun is glowing.

It's that hot.

NARRATOR:
If we could open up a space

between the mantle and the core,
this is what it might look like.

LATHROP:
Just inside the mantle,

liquid metal meets the mantle.

There's probably, you know,
a bit of a mushy zone,

where there's
liquid metal mixing in

with the last bits
of mantle material.

And then inside of that

is just this vast, deep ocean
of liquid metal,

which is red-hot, flowing,

there's all this
churning motion,

and probably things
that are analogous to clouds,

in the sense of bits that
are more dense and less dense

mixing about
as the core convects.

NARRATOR: Seismologists can see
what the outer core looks like

because seismic waves
bounce off its liquid surface.

And scientists like Dan Lathrop

are discovering what's going on
inside the core

by measuring the powerful
electromagnetic energy

it produces...
the Earth's magnetic field.

LATHROP: If you look at
the pattern of magnetic field

on the outside of the Earth,

it's quite clear that
that pattern is slowly moving

and slowing changing in a way
that would be easily described

by it rising from a liquid metal

that's also slowly moving
and slowly convecting.

NARRATOR:
The Earth's magnetism

has been known about
for more than 1,000 years.

And for centuries,
explorers and sailors

have kept detailed records of
our moving magnetic North Pole.

We now know that birds
and animals use it to navigate

on their epic migrations
across continents and oceans.

By the 1950s,

scientists understood
that something made of metal

was responsible
for the magnetic field.

It was the Earth's core.

Dan Lathrop wants to know how
the field could be generated,

so he's built
a model of the core,

a sphere filled
with liquid metal.

Not iron, but sodium.

Iron would be too heavy
and dangerously hot.

But sodium isn't perfect either.

Well, sodium has its pros
and cons, without a doubt.

It's a very good
electrical conductor...

an excellent
electrical conductor...

so it gets us closer
to being like a planet

in the laboratory experiments.

The cons are,
it's a reactive liquid.

It is flammable,
burns readily in air,

and also reacts violently
with water.

NARRATOR: With the 13 tons
of sodium safely sealed inside,

the 10-foot sphere
starts to spin

to re-create
the Earth's rotation.

Heaters keep the sodium molten.

Minutes later,

magnetic fields spill from
the sphere in all directions.

Lathrop's experiment confirms

the way the Earth's
magnetic field is generated.

Driven by the heat,

the convection currents
in the core

combine with
the Earth's rotation

to create a giant dynamo.

LATHROP: The dynamo is like
an electrical generator,

but it's being driven
by the motions

of the liquid outer core.

And that churning motion,

sort of turbulent convection
in the core,

couples with the magnetic field

to continuously regenerate
the magnetic field.

It's like the turning motion
of the generator,

in this case then, is
the churning of the convection.

NARRATOR:
The magnetic field

is much more
than a geological curiosity.

It's vital to life on Earth.

The field protects us from
our closest, deadliest enemy...

the sun.

A giant nuclear reactor,

enormous storms
rage on its surface.

These storms fling
lethal radioactive particles

into space.

This is the solar wind, and
Earth lies right in its path.

But like a stone in a stream,

the Earth's magnetic field
parts the flow of radiation,

diverting it around the planet.

We sit in a protective pocket
of magnetism...

the mystery of life

made possible by the mysterious
core of the planet it inhabits.

The Earth's magnetic field
is absolutely critical

for Earth
to be a habitable planet,

in the sense that
the quite violent radiation

coming from the sun stream
around the outsides of a bubble

formed around the Earth
by the magnetic field.

So the magnetic field
extends a sort of shield,

the magnetosphere, which
protects us and the atmosphere

from most of the radiation.

If that weren't there,
the solar radiation

would be constantly
bombarding the atmosphere,

actually eating away
at the atmosphere,

and some of it then directly
making it down to ground level.

NARRATOR: About 40,000 miles
above the poles,

the charged solar particles

meet the outer reaches
of the magnetic field.

Here, some are diverted down

toward the Earth's
magnetic poles,

where they create spectacular
auroras that glow in the sky.

These dazzling displays happen

when the particles
slam into gas molecules

in the Earth's upper atmosphere.

Although beautiful,

these are a sign
of a ferocious battle

between the Earth's core

and an invading stream
of solar radiation.

Our magnetic field
protects us from other dangers,

not just from the sun.

Lethal cosmic rays
made of radioactive particles

permeate deep space.

Down on Earth,
we're unaware of them.

But up in space,
it's a different story.

On July 20, 1969,

Neil Armstrong was the first man
to set foot on the moon.

It was one of humankind's
greatest achievements.

But on their way to the moon,

Armstrong and co-pilot
Buzz Aldrin

saw flashes of light inside
the darkened Apollo 11 module.

Bizarrely, they even saw the
flashes with their eyes shut.

When they returned to Earth,
they reported what they saw.

NASA scientists were mystified.

Six years later, they came
to believe these light flashes

were the result
of high-energy cosmic rays

penetrating the spacecraft
and the crew members' eyes.

Armstrong and Aldrin
were exposed to these rays

because the Apollo craft

was near the edge
of the safety shield

of the Earth's magnetic field.

MAN: 3, 2, 1.

And liftoff of Discovery.

NARRATOR:
In the years since,

at least 39 astronauts

have developed
some kind of eye cataract

a few years after exposure
to this dangerous radiation.

Without the Earth's
magnetic field,

we would all be exposed
to these dangers.

And it's the core
that is our great protector.

We know the magnetism comes from
the rotation of the core

and the turbulence
of the molten metal within it.

But how can we work out exactly
what's going on inside the core?

Peter Olson is one scientist
who's devised an experiment

that could offer an explanation.

Well, what we have here

is nothing more than a large
tank of water on a turntable.

And what it's
intending to simulate

is the Earth's outer core.

And we're going to inject
some heavy dye

into this big tank of water,

and we're going to see
the effects of the rotation

on the turbulence.

There's a turbulent plume

trying to sink
to the bottom of the tank.

But it starts to feel
the effect of the rotation,

and you can see it gets twisted
up into kind of a helix.

And it's this helical type
of flow in the Earth's core

that we think is so critical

for generating
the Earth's magnetic field.

Ordinary turbulent motions

don't have this kind
of helical structure to them.

But by virtue of the effect
of the Earth's rotation,

the turbulence in the core
is made helical.

NARRATOR:
These helical columns

might explain
the Earth's magnetic field.

They represent
liquid-iron columns,

which could work like the wire
coils inside an electromagnet.

As they move with the Earth's
rotation, they create magnetism.

2,500 miles
below the Earth's surface...

could there really be
molten columns of liquid iron

hundreds of miles high?

OLSON: As a consequence
of this turbulent motion

of the liquid iron,

electric currents
are flowing in the core.

And the geomagnetic field
that we see at the surface

is actually the result
of these electric currents.

So there is no bar-magnet
or permanent-magnet effect

of any significance
inside the core of the Earth.

The magnetic field there is
produced by electric currents.

NARRATOR:
This delicate feedback system

makes the core
seem extremely fragile.

Without heat or rotation,
it wouldn't work.

To demonstrate,

Olson simply switches off
the tank's rotation.

The water keeps moving,
but as it slows down,

the convection currents
gradually collapse.

If this happened in the core,

the Earth's magnetic shield
would soon disappear.

Deep inside the Earth's core,

something mysterious
is happening.

Swirling currents
of molten metal

are creating a magnetic field
that envelops the planet.

We depend on this field

to protect us
from deadly solar radiation.

But scientific data

shows that magnetic field
is weakening.

Over the past century,

the strength of the planet's
magnetic field

has declined by nearly 10%,
and scientists aren't sure why.

During most
of mankind's history,

the magnetic field
has been very strong.

And now it's weakening.

LATHROP:
The Earth's magnetic field

has been studied
for about 160 years.

And what people see is
that the magnetic field

has slowly and steadily dropped
in its strength.

NARRATOR: In one region,

the magnetic field
is a third weaker.

It's here
over the Atlantic Ocean,

just off the coast of Brazil.

It's known
as the South Atlantic Anomaly.

This disruption
in the magnetic field

stretches a quarter
of the way around the globe,

and it's growing.

Every day in this area,

cosmic radiation reaches closer
to the Earth's surface.

This protection that we get
from the solar radiation

from the magnetic field
is already weaker in that patch,

so it already
has implications...

mostly for astronauts
and people who run satellites.

OLSON:
It's really come into prominence

since the advent of long-term
orbiting spacecraft.

For example,
the Hubble Space Telescope

has had enormous problems
over the years

as it passes through
the South Atlantic Anomaly.

NARRATOR:
The problem is so bad

that when the billion-dollar
Hubble Space Telescope

is above the area,

vital instruments are routinely
shut down for protection.

[Radio chatter]

And near the core
under the South Atlantic,

something even stranger
is happening.

The magnetic field here
hasn't just weakened,

it has totally reversed.

LATHROP:
If you look at what

the magnetic field would be
at the edge of the core,

the magnetic field down there

has already reversed
in that patch.

Now, this could be a sign,

if this becomes
deeper and broader,

that we're headed
toward a reversal.

NARRATOR: A reversal is
a total change in polarity

of the Earth's magnetic shield.

The North Pole flips
to the south,

and the South moves north.

LATHROP: What a reversal is,

is when those
North and South Poles reverse

so that you have
a long, steady period

where they're
in one orientation,

and then there's a reversal

and then a long, steady period
in opposite reversal.

NARRATOR:
Reversals have happened before.

We know this because,
when lava cools,

it preserves evidence
of the Earth's magnetic field.

Crystals inside the molten lava
line up with the field.

When it solidifies,

it creates a record
of its strength and direction

at that exact moment in time.

Studies of
prehistoric lava flows

indicate that the last reversal
happened 700,000 years ago,

when our apelike ancestors
roamed the Earth.

You might think that,
if the field is so stable

that it can persist
for billions of years,

why should it
suddenly decide to change?

But it does.

We know that the Earth's
magnetic field

has reversed
many hundreds of times.

What we don't know is
when will it do it next?

NARRATOR: Neither do we know
what will happen when it does.

The weakening magnetic field
and the South Atlantic Anomaly

are the signs that we're about
to experience the next reversal.

It could happen
within the next 1,500 years.

OLSON: The rate of decrease
is about 6% per century.

Now, that doesn't sound
like very much, perhaps.

But in geologic terms,
that's extremely rapid.

NARRATOR: No one knows
what a reversal will mean

for life on Earth.

But while
the magnetic field reverses,

we would lose its protection
for several months.

Solar radiation would penetrate
our electrical systems.

Surges would overload
the world's power grids.

At the same time,

bats, birds, and whales
could become disoriented

as their internal navigational
systems are scrambled.

There could even be
an increased incidence of cancer

as solar radiation
attacks our cells' DNA.

We might see auroras appearing
all over the planet.

Even over our major cities.

No one knows exactly when
the next reversal will happen,

but the answer could lie
even deeper inside the Earth

in the inner core.

It's the least understood,
most remote,

and inaccessible place
on the planet.

And somewhere
in this hidden, hostile world

lies the key
to the Earth's future.

The inner core is
a rotating sphere of solid metal

floating inside
the liquid outer core.

Billions of amps of electricity
leap across its surface.

Hotter than the outer core,

the inner core's heat
is the ultimate driving force

behind the Earth's
magnetic shield.

OLSON:
The pressures are so high

towards the center of the Earth

because of the overlying weight
of so much material,

that despite the fact
that it's hot,

the material is still solid.

NARRATOR: Seismic studies
tell us something else

about the inner core...

slowly but surely,
it's growing.

Every year,
it expands by one millimeter

as the planet loses heat.

Nobody has ever seen this
process with the naked eye.

But in the lab, scientists can
use their imagination

to show something similar.

LATHROP:
So as the Earth cools,

the inner core grows
by iron crystallizing onto it.

We could imagine
what that looks like

by looking at ice crystallizing
onto this cool sphere.

A lot of people
who think about the core

sit around and argue about,
what's that surface like?

Is it rough? Is it smooth?
Is it mushy?

What we know is that,

from the earthquakes
passing through,

if it is rough,

the thickness of that
is less than a mile or so.

But that still leaves
lots of room

for mushy zones
or cavernous pits

and little mini mountains.

We really have no idea
what that surface looks like.

But if you look at any other
surface on the Earth,

on other planets
elsewhere in the solar system,

they're all rough.

Even the surface of the ocean
is rough,

of course,
moving about with the waves.

And so my expectation is

that things are quite rough
and quite complicated.

NARRATOR: Exactly how
rough and complicated

is open to debate.

Dan Lathrop believes
the inner core's surface

is probably covered in a forest
of metallic projections.

They're called dendrites.

LATHROP: There's most likely
a sort of rough surface

of these iron crystals,
perhaps dendrites poking out.

And the whole core itself

has a sort of crystalline order
to it.

So while it's roughly spherical,

it has crystalline bits
growing out from it,

continuously growing larger.

NARRATOR: As the core cools,
the dendrites grow.

It's a sign that heat is
constantly being transferred

from the inner
to the outer core.

The Earth is slowly cooling,
just from its origin.

And whenever you have something

which is hotter on the inside
and colder on the outside,

it tends to get
flows going, vortices.

You know,
think of them sort of like

big, tumbling,
cloudlike motions,

but it's in the liquid metal
in the core.

NARRATOR: This heat transfer is
fundamental to life on Earth.

It powers the outer core
and the Earth's magnetic shield.

But it won't last forever.

With planet Earth losing heat
every second, every day,

one thing is certain...

the inner core
will keep growing and cooling.

In the distant future,

the whole core
will freeze solid.

For life on Earth,

the consequences of that
are unthinkable.

The inner core of planet Earth
is a mysterious place,

hotter than the surface
of the sun,

yet it's solid metal.

The core radiates
incredible heat energy outward.

At the same time,

it crushes everything
down around it

with intense gravity.

There's no way to see it
or sample it.

How did it get there?
Where did it come from?

There are clues.

The Earth shares its origins
with the other rocky planets...

Mars, Venus, and Mercury.

In the beginning,
just after the sun lit up,

before the planets existed,

great clouds of cosmic debris
orbited the newly ignited star.

These early building blocks

crashed into each other
with massive force.

The bigger the objects became,

the greater
their gravitational pull,

until eventually
whole planets formed.

LATHROP:
When a planet forms,

it forms from a hodgepodge

of all sorts
of different materials.

And so the heavier bits would
tend to fall under gravity

and accumulate
into the interior of the Earth.

We know
that the bits of material

that made up
all of the inner planets

had quite a bit
of iron in them...

just raw, metallic iron.

And that would tend
to sink down eventually

to form this massive core
of the Earth.

NARRATOR: The solar system
is now complete and stable,

but the process of formation,
called accretion,

is not quite over.

The spare parts left over

from the creation
of the solar system...

asteroids,
comets, meteorites...

still orbit the sun
and still crash into the Earth,

like the one
that created this...

Meteor Crater in Arizona.

It was formed by an impact
50,000 years ago.

And for cosmochemist
Meenakshi Wadhwa,

it offers a glimpse
of the forces and the materials

that created the Earth's core.

WADHWA: So Meteor Crater
that you see here

was created by the impact
of an object

probably that was
about 300, 400 feet across.

And this was an event that was
a sudden, catastrophic event.

A lot of energy was released...

something like
20 megatons or so.

NARRATOR:
Lmagine a planet growing

from billions of impacts
like this one,

each one delivering
iron, nickel,

and the other elements
that make the world around us.

They also delivered an enormous
amount of heat energy.

WADHWA: You can see
that there were large blocks

that were ejected
out from the crater,

and there were
actually material

probably tossed out to hundreds
of miles from the crater

as a result of the impact.

NARRATOR:
The impact here was so powerful,

it vaporized the meteorite.

But a few fragments survived.

So this particular
meteorite is...

It's called
a Canyon Diablo meteorite,

and it's an iron-rich meteorite
which was part of the impactor

that created Meteor Crater.

It's very difficult, of course,

to actually sample a piece
of the Earth's core,

but these meteorites
right here provide us a window

into looking
at planetary interiors.

And you can
actually learn something

about core-formation processes

by looking
at iron-rich meteorites.

NARRATOR: Close up,

you can see the crystalline
structure of the metal

that exists right
at the heart of our planet,

a planet that's unique
in the solar system.

But what makes Earth so special?

If the other rocky planets
were made the same way,

how come
they're so different today?

What happened to them

might shed light on the future
of our own planet.

Scientists look to them
for clues

that can tell them more about
the fate of the Earth's core.

And the planet that
interests them most is Mars.

It's our nearest neighbor.

Like Earth, water once flowed
on its surface.

It had a thick atmosphere.

But that was
billions of years ago.

Today, the planet
is a frozen desert.

Most of its water and atmosphere
have vanished.

And even though Mars
has a metal core,

its magnetic field is tiny.

Are these conditions
a coincidence?

Or is Mars
a vision of Earth's future?

MAN:in NASA's
Mars Global Surveyor.

NARRATOR: In 1996, NASA launched
the Mars Global Surveyor.

Its mission... to unlock
the secrets of the red planet.

MAN:as America begins its
journey back to the red planet.

NARRATOR:
But in the process,

it unlocked some
of our own planet's secrets,

shedding new light on the very
center of the Earth...

the inner core.

The Global Surveyor's data
astonished scientists.

It showed Mars' magnetic field
is very weak,

but Mars' crust is
intensely magnetized.

The implications
for our planet are immense.

Like Earth, Mars once had
a powerful magnetic field.

But at some point, the Martian
core cooled and froze,

and its magnetic field
collapsed.

The question is,
could it happen to our planet?

Mario Acuna was
one of the scientists

who built the magnetic sensors
that gathered the Mars data.

He used it to create a map
of Mars' magnetized crust.

He discovered that in one area,
there is no magnetism at all.

And it corresponds with
a particular physical feature.

One of the things
that we observe

is this very large hole in Mars,

if we want
to call it a hole.

It's really the remnants
of a gigantic impact

that took place very early
in Mars' history.

NARRATOR: This hole is
an enormous meteor crater.

It was clear
that the rocks here,

unlike those in the rest
of Mars' crust,

hadn't been magnetized.

So the crater must have formed

after Mars' core
stopped working.

Scientists think
the meteor impact here

released so much energy,

it liquefied the planet's crust
at the point of impact.

Crystals in the cooling lava

would have recorded
the surrounding magnetic field,

just like they do on Earth.

But in the gigantic crater
on Mars,

the rocks bear no record
of being magnetized.

Scientists theorize

that's because the magnetic
field no longer existed

when the impact occurred.

The continent-sized crater was
created 4 billion years ago.

It means the dynamo
in Mars' core

stopped working when the planet
was in its infancy.

DR. ACUNA:
For the first time,

we could time
when the dynamo disappeared.

And since Mars was formed
only 41/2 billion years ago,

that means
that the dynamo only lasted

a few hundred million years.

NARRATOR: The reason
for Mars' premature death

lies in its size.

Mars is
half the diameter of Earth,

so it cooled more quickly.

Its core froze,

and its magnetic shield
collapsed.

The fate of life on Mars
was sealed.

The planet lay exposed
to the solar wind.

Its atmosphere and water
eroded away.

DR. ACUNA: The fact that
the magnetic field disappeared

had a tremendous effect
on the loss of water by Mars.

We are looking for something
like 1,500 feet of water

over the entire planet Mars
to have disappeared from Mars.

NARRATOR:
Earth is much larger than Mars,

so its core is still hot,
still working.

But the lesson of Mars
is unavoidable.

Eventually,
Earth's own core will cool

until the convection columns
inside the outer core collapse,

and then our magnetic shield
will come down.

Without it, solar radiation
will strip away

both our atmosphere
and liquid water.

Then Earth will become
a dead and desolate place.

But we don't need
to panic just yet.

The extreme temperatures
in the inner core

suggest we have
plenty of time left,

perhaps even billions of years.

Nearly 4,000 miles
from the surface,

we have reached
our destination...

the very center of the Earth.

This is the hottest part
of the planet.

Temperatures reach
12,000 degrees,

hotter than the surface
of the sun.

And with no gravity,
it's like nothing else on Earth.

The very center of the Earth is

probably the most un-Earthlike
place on the planet,

in the sense that gravity
gets weaker as you go down,

and when you hit the center,
there's no gravity left.

There's no direction
which means down.

Gravity is absent.

The temperature is
the hottest spot on the Earth.

And so it's this sort
of white-hot, gravityless,

very high-pressure...
just crushing pressures

of all of the weight
of the rest of the Earth

all pushing down on you.

So it's extremely inhospitable

and extremely strange
at the same time.

NARRATOR:
The world beneath our feet

may seem like an alien place,

but our journey has shown

it's very much
part of life aboveground.

Everything about it
is just right.

The Earth spins
at precisely the right speed,

and it's exactly the right size

to allow some heat loss
from the core, but not too much.

As a result,
we have our magnetic field.

The mantle is just mobile enough

to allow currents of heat
to move upward

so we have our continents
to live on.

And our gravity
is just the right strength

to bind our atmosphere
and oceans to the surface.

From the crust to the core,

every layer, every rock,
every piece fits together

to make life
upon the surface possible.

The secret of all life
as we know it

lies deep inside planet Earth.