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The deepest part of the ocean is the Mariana Trench, an oceanic trench located in the Pacific Ocean near the island nation of Guam. At its deepest point, known as the Challenger Deep, the Mariana Trench is almost 7 miles (11 kilometers) below sea level. Just to put that in a frame of reference, if someone were to shave Mount Everest off the surface of the Earth and drop it into the Mariana Trench, it would disappear, buried in over 1 mile (1.6 km) of water.
As can be imagined, the pressure in the Mariana Trench is extreme: about 1,000 times the atmospheric pressure at sea level. Organisms like humans, who are accustomed to life at sea level, would implode within fractions of a second if exposed to that depth, and the creatures that live in the Mariana Trench demonstrate a number of unusual adaptations which help them cope with the pressure. Algae, bacteria, marine worms, and an assortment of other unusual creatures live in the total darkness and extreme cold, interrupted only occasionally by survey submarines sent to explore the trench for science.
This incredibly deep ocean trench has formed at what is known as a convergent plate boundary. The deepest part of the ocean is formed by the subduction of the Pacific Plate under the Philippine Plate. To get an illustration of what the trench looks like, a person can slide one of his hands under the other. Right along the boundary where the hands meet, a notably deep trough is formed; if this is magnified significantly, it can provide an idea of what this part of the ocean looks like.
The first survey of the Mariana Trench was undertaken in 1951 by a British team on board the Challenger II. Since the team discovered the deepest point of the trench, the Challenger Deep was named after them. A United States Navy bathysphere visited the bottom of the trench in 1960 with two men on board. Oceanographers liken this expedition to the first moon landing, because of the immense amount of preparation and danger involved, and some like to point out that more is known about the surface of the moon than the deepest part of the ocean.
The Mariana Trench isn't the only deep ocean trench, although it is the deepest, extending to twice the average depth of the world's oceans. Given the extreme conditions at the Mariana Trench, it's unlikely that most people will be spending any time there, but if they do, they will be able to see a fascinating array of marine organisms that have only been seen by a handful of human beings.