In fact, for most of history men sailed on the surface of the ocean and submarines navigated the topmost few hundred meters, but the only ships that visited the ocean floor were those that sank, never to return.
Part of the reason for this lack of direct knowledge is sea pressure. The weight of a column of seawater increases by 44 pounds per square inch (psi) (3.1 kg/m2) for every 100 feet(30.5 m) of depth. A mere 100 feet of seawater, then, will exert a pressure of 44 psi over each of the 144 square inches (929 cm2) in one square foot (0.9 m2)for a pressure of 6,336 pounds (2,877 kg). In other words, a vessel with only one square foot of hull would have three tons(2.72 tonnes) of force acting against it at a depth of only 100 feet. The Challenger Deep, at a depth of about 36,000 feet (10,973 m) experiences a pressure of almost eight tons per square inch (11,249,112 kg/m2). Pressure alone is sufficient to keep people from venturing to these depths without taking extraordinary measures. Add to this equation the necessity to breathe, maneuver, and return to the surface and one begins to understand why the ocean depths were not visited until 1960 and why, even today, they are known chiefly only by indirect means.
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