Pressure is force applied to an area. In the case of ocean water, the force depends on the water's mass and the acceleration due to gravity. The mass, in turn, depends of the water's volume and density. The two factors of volume and area partially cancel each other out, leaving as the only independent variable the water's height, which is the ocean's depth. The farther down you go in the ocean, the greater the water pressure that will act on you.

Multiply the ocean floor's depth, in feet, by 0.3048 to convert it to meters. The ocean's deepest point in the Marianas trench, for instance, is 35,839 feet below sea level: 35,839 x 0.3048 = 10,924 meters.

Multiply this answer by the the density of sea water, which is approximately 1,025 kilograms per cubic meter: 10,924 x 1,025 = 11,197,100.

Multiply the result by gravitational acceleration, which is a constant equal to 9.81: 11,197,100 x 9.81 = 109,843,551 pascals, or about 110 megapascals. This is, for comparison, more than a thousand times atmospheric pressure.
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