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.
At a hundred feet, the pressure can cause nitrogen to begin dissolving in a person’s blood. That can make a diver drowsy and disoriented. At greater depths, oxygen begins to dissolve, causing vomiting, seizures, and even death. So only the most experienced divers go very deep, and then only with special equipment.
To go deeper, then, you need protection. A typical modern military submarine might reach about half a mile before the pressure crushed its hull. A small research sub can descend several miles, where the pressure is a few tons per square inch. Robotic subs can go deeper still.
The deepest spot in the oceans is the Challenger Deep, in the western Pacific, with a depth of almost seven miles. The pressure there is eight tons per square inch. That means an unprotected adult would be squeezed by the weight of about 50 fully fueled jumbo jets. So only three people have ever ventured into this realm — protected from the crushing pressure of the ocean depths.