A Steller sea lion breaks the surface of the wave-tossed Gulf of Alaska 40 miles south of Kodiak. She takes a few deep breaths and floats at the surface, resting. She just made a dive of 300 meters, more than a thousand feet, and was underwater for ten minutes catching cod on the sea floor.
The pressure at that depth is about 450 pounds per square inch. It's remarkable that a sea lion can withstand such pressure, and transition between that pressure and the surface. Terrestrial animals like us experience a steady pressure of one atmosphere - about 15 pounds per square inch - the weight of the air above us. But water is heavier than air, and every 10 meters of depth adds another atmosphere of pressure. At 300 meters below the surface, that sea lion experienced 30 atmospheres of pressure.
Fish and Game researchers found that sea lions in PWS were making dives deeper than 400-meters, and one female went as deep as 600 meters, over a-third of a mile, where the pressure tops 900 pounds per square inch. Specially designed bathyscapes can reach such depths, but that's about twice as deep as a modern military submarine can go.