Challenger Deep - the Mariana Trench
>BR> 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. They touched bottom at 35,813 feet.

That means, while they were parked on the bottom in the bathyscaphe, there were almost seven miles 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!

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.

So, How Do They Know?

In 1984 the Japanese sent a highly specialized survey vessel out the 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 (that the human ear can't hear) 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.