How Far To The Bottom Of The Ocean?

Mar 31, 2012 By Anita R
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Even SpongeBob may not be able to tell us precisely where the bottom of the ocean is. It is an irony that we know more about the surface of the moon than we know about Earth’s ocean floor.

70% of Earth’s surface is covered by water. Very little or no light penetrates beyond a few hundred feet in water. Besides, trillions of pounds of water make the pressure extremely heavy at the bottom of oceans, causing scientific instruments sent below to be crushed. Studies over the last couple of centuries have however given us clues that mountain ranges and valleys far higher and deeper than even Mt Everest can be found at the bottom of the ocean.

Earlier this week, filmmaker James Cameron became the first solo diver to go down to Challenger Deep in the Mariana Trench, regarded today as the deepest point on Earth. How do we know that there isn’t somewhere deeper? How are ocean depths measured? Let’s find out.

In the land of buried mountains

Bathymetry or the science of underwater topography involves the study of depth of oceans. A long time ago, sailors measured ocean depth, by using heavy rope or cable marked off in fathoms - a nautical measure of depth. The depth would be noted by how much line was put out until the weight hit the bottom of the ocean or sea floor. You may have figured this method has its faults. It can measure the depth of only at a single point; is inefficient and subject to errors due to the ship and current movements of the line.

As technology developed so did the methods used by oceanographers.

Yell and you shall know

If you have stood on the edge of a cliff, canyon or mountain and yelled into the valley, you may have noticed your voice echoes back to you. Scientists have studied this phenomenon and figured that echo happens because sound travels as waves. Sound is produced by vibration of particles present in the medium or material though which it travels. We hear sounds when the vibrations reflect back into our ear.


Armed with the knowledge of sound, its speed through water and reflection from solid surfaces, bathymetrists today use sonar (Sonar – SOund NAvigation and Ranging) to send sound signals from a boat or sea to the bottom of the oceans. They measure the time it takes for the sound to return back to the boat. Sophisticated satellite GPS -Global Positioning Systems, help to pinpoint the exact geographical location of the sound signals of the measured ocean depths.

Today the deepest ocean discovered is in the southern part of the Pacific Ocean in the Mariana Trench. The Challenger Deep has been established as the deepest point in that trench and is 6 1/2 miles deep! Compare that with Mt Everest which is 5.49 miles tall. As technology becomes more sophisticated, there may yet be another point that is deeper than Challenger Deep. And if so, we are yet to discover it.