|MadSci Network: Environment & Ecology|
Most parts of the ocean are about 5 kilometers (3 miles) deep. However, deep ocean trenches can be as much as 11 km (7 miles) deep. The deepest point in the world's oceans is the Challenger Deep, in the Marianas Trench; its depth is 11.03 km (36,200 feet). In 1953, the bathyscaphe ("bathy" = deep, "scaphe" = ship) Trieste descended to 35,800 ft (10.91 km) in the Challenger Deep. The Trieste was manned by Jacques Piccard and Don Walsh; it was built by Jacques and his father, Auguste.
So the answer to your second question is that man has been almost to the deepest point in the sea.
What's the pressure there? You can find the pressure by using the hydrostatic relation:
p = rho g zwhere p is the pressure, rho is the density of seawater (about 1030 kg/m3), g is the acceleration of gravity (9.81 m/s2) and z is the depth. I get 1114 bars -- that's over 1000 times the atmospheric pressure at sea level.
I got much of this information from a web page on deep-sea exploration. Jacques Piccard has apparently written some books about his experiences: The Sun Beneath the Sea and (maybe) Seven Miles Down. (I saw a reference to the latter on this web page, but can't find it in the Library of Congress. There's also a biography of the Piccards by Alida Malkus entitled Exploring the sky and sea; Auguste and Jacques Piccard.
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