Deepest
Place in the Ocean...
CHALLENGER DEEP
Challenger Deep
got its name from the British survey ship Challenger II, which
pinpointed the deep water off the Marianas Islands in 1951. Then
in 1960, the US Navy sent the Trieste
(a submersible
- a mini-submarine designed to go really deep) down into the
depths of the Marianas trench to see just how far they would go (read
the original press release). They touched bottom at 35,838 ft/10,923m.
That means, while they were parked on the bottom in the bathyscaphe,
there were almost seven miles/11km of water over their heads!
If you cut Mount
Everest off at sea level and put it on the ocean bottom in the Challenger
Deep, there would still be over a mile of water over the top of it!
Hydrostatic Pressure
When you get
into the ocean (or any body of water) and you start diving down from
the surface, the deeper you dive the more water is over the top of you.
The more gallons of water you put between you and the surface of the
ocean, the greater the pressure is on your body because of the weight
of the water over the top of you. This pressure is called hydrostatic
pressure.
You can really
get a sense of hydrostatic pressure when you go into a swimming pool
and dive all the way to the bottom of the deep end. You'll feel the
hydrostatic pressure against your ear drums, like they're being squeezed
or pushed in. Well, you can imagine how incredible the pressure must
be in the Challenger Deep with almost seven miles of water overhead
- it's 16,000 pounds per square inch!
The Trieste in
1960. |
Plate
Tectonics and the Subduction Zone
So how come
the Challenger Deep is so deep? Well, the earth's crust isn't one solid
piece of rock, it's really pretty thin, like the shell of an egg is
compared to the size of the egg. In fact, it's made up of huge
plates of thin crust that "float" on the molten
rock of the earth's mantle. While floating around on the mantle the
edges of these plates slide past each other, bump into each other, and
sometimes even crash. The oceanic crust is much heavier than the continental
crust so when the plates crash into each other, the oceanic plate plunges
downward toward the molten mantle, while the lighter, continental plate
rides up over the top. The forces driving the two plates together are
really intense so the underlying oceanic plate (the subducted
plate) creates a trench where it drags the edge of 
the
continental crust down as it descends underneath (check out the picture
at left).
This is what's
happening on the bottom of the Pacific Ocean off the Marianas islands.
The really deep part of the ocean is in the bottom of the trench created
by the subducting ocean
crust.
How Do They Know?
In 1984 the
Japanese sent a highly specialized survey vessel out to the Marianas
Trench and collected some data using a piece of equipment called a narrow,
multi-beam echo sounder.
What an echo
sounder does is send high frequency sound waves (outside the range of
human hearing) through the water down to the ocean bottom. Sound waves
will travel through water, even faster than they travel through the
air, and bounce off solid objects, such as the ocean bottom. The echo
sounder measures precisely how long it takes for the sound waves to
be returned to the surface and determines the depth based on the rate
of return. These soundings are plotted on a graph by a computer to make
an "echo map" of the ocean bottom. 
The deepest
measurement of the Challenger Deep currently available was taken by
the Japanese and was found to be 35,838 feet.
See
the different ways scientists study the ocean
Find out what creatures live in the ocean depths, visit the Deep
Ocean Creatures page.
Learn
about life
zones
of the ocean.
Meet
a scientist who studies the ocean's deep, right here at Extreme Science.
