How the Earth Was Made

>> Earth, a 4.5-billion-year-

old planet, still evolving. As

continents shift and clash,

volcanoes erupt, and glaciers

grow and recede, the Earth's

crust is carved in numerous

and fascinating ways, leaving

a trail of geological

mysteries behind. In this

episode, the Marianas Trench,

the deepest point on Earth, is

explored. Its sheer walls cut

seven miles into the Pacific

Ocean. The mystery of what

created this deep, dark chasm

takes science detectives on

some of the most dangerous

dives ever attempted, deep

into the abyss. Scouring the

ocean floor, scientists

uncover a strange,

undersea world of fiery

mountains, bizarre mud

volcanoes and the largest

geological structure on Earth.

Discoveries from this unique

underwater world will

revolutionize our

understanding of the powerful

forces that shape not just the

trench, but the Earth itself.

S01x02 The Deepest Place on Earth Original Air Date on February 17, 2009

Hidden deep beneath the waves

of the western Pacific lies

the Marianas Trench, the

deepest point of all the

oceans.

The first step on the journey

of what created this

mysterious scar in the Earth's

crust, and how it continues to

mold the planet, takes us

back to 1872, when a British

research vessel, HMS

Challenger, set out on the

first ever mission to map the

ocean floor.

>> Throughout most of recorded

history, men had just assumed

that, beyond a certain level,

the sea was pretty flat,

pretty dead, pretty lifeless.

They weren't expecting to find

anything very interesting.

>> For four years, the

Challenger crisscrossed the

oceans, covering 70,000 miles,

a third of the distance to the

moon.

The crew plumbed the depths

every 140 miles, using a total

of 249 miles of rope, and

hundreds of pounds of

lead weight. It was tedious,

backbreaking work, but at the

time, it was the only way to

measure the depth of the ocean

floor.

When they got to the western

Pacific, 200 miles off the

island of Guam, the crew

routinely lowered the rope for

a measurement.

But the weight kept on

dropping and dropping.

>> It's a big surprise! Nobody

thought the ocean was this

deep. So all of sudden we've

got scientists saying, "Why

is that?"

>> Eventually, the weight

struck the bottom at 4,475

fathoms, nearly five miles

beneath the ocean's surface.

>> The scientists would be

going, "Wow, we've found

something and what does it

mean? Is it a little hole? Is

it a big hole? What kind of

feature is it down there?"

There--there's a whole lot

of questions you get when you

find this one spectacular

reading.

>> The Challenger expedition

marked the birth of modern

oceanography, and provided the

first crude map of the ocean

floor.

It showed how the ocean floor

gently slopes away from the

land, and then plummets

thousands of feet into vast

flat plains. But the western

Pacific is different. It drops

off again, into the five mile

deep hole, a hole that blew

right out of t water the

long-held belief that the sea

floor was flat and

featureless.

And it spawned a mystery,

because nobody could

understand how this strange

underwater feature came about.

It would be 75 years before

any answers emerged. It took a

revolutionary new technology,

sonar, to push the

investigation forward to the

next crucial stage.

Sonar was first developed in

the early 1900s and then

perfected during the 1940s to

detect submarines lurking in

the deep.

The system works by pumping

sound waves through the water.

The waves bounce off solid

objects and are reflected back

to a detector. By measuring

the time it takes for the

sound waves to bounce

back, scientists realized they

could build a remarkably

accurate picture of the world

beneath the waves.

>> The world's major navies

spend a lot of time and effort

developing submarine hunting

technology, then the

hydrographers discover that

you can use this to chart the

bottom of the sea and it's an

awful lot cheaper and easier

than using large numbers of

sailors pulling on ropes.

>> In 1951, a British Navy

research ship returned to the

deep hole found by the

Challenger expedition.

But, this time, they were

armed with sophisticated new

sonar equipment.

And the results were amazing.

Detailed sonar maps revealed

that the deep hole in the

Pacific Ocean floor isn't a

hole at all, but part of a

massive trench, 30 times

deeper than the Empire State

Building is high.

It runs twice the length of

California, 1,500 miles from

the southeast of Guam to the

northwest of the Mariana

Islands.

>> People were probably

astounded by what they were

seeing, because, clearly, the

ocean floor had enormous

changes in relief. It was very

mountainous in some places,

had great deeps in other

places. To a geologist, this

would be extremely exciting.

Even within the trench itself,

there are remarkable

variations. At its southern

end lies the greatest surprise

of all.

The sea floor drops down

another two miles to its

lowest point, a staggering

seven miles beneath the waves.

Scientists had discovered the

deepest part of the oceans.

Even today, it is the lowest

known point on the planet.

They named this part of the

trench the Challenger Deep, in

honor of the ship that

discovered it.

>> To get a sense of just how

deep trenches are, if we take

the heig

we would still have about a

mile of water above us before

we get to the ocean surface.

>> But how the Marianas Trench