A sperm whale can dive thousands of feet deep to catch its fill of squid. The pressure causes its rib cage to buckle inward and its lungs to collapse. Yet the whale survives just fine because that’s what’s supposed to happen. Not so with people, though. Depths of a hundred feet can cause problems, and anything below a few hundred feet is fatal.
The pressure is caused by the weight of the water above. Every 33 feet in depth adds the equivalent of the pressure of the atmosphere at sea level — 14 and a half pounds per square inch.