Why does deep water crush submarines?

The answer

Water pressure increases by about 1 atmosphere (14.7 psi) for every 10 meters of depth because the weight of all the water above presses down. At the deepest point in the ocean, the Mariana Trench (about 11,000 meters), pressure exceeds 1,000 atmospheres - over 16,000 pounds per square inch. This is enough to compress a styrofoam cup to the size of a thimble. Submarines must be engineered with thick, precisely shaped hulls to resist this immense inward force.