Divers "equalize pressure' in only one respect. Our middle ears, where our ear bones sit, are air-filled cavities, closed to the outside by the ear drum. When a diver dives, the increasing water pressure pushes the ear drum inward, causing pain. With practice a diver can learn to manipulate (by moving their jaws, for example), a tube (called the eustachian tube), which connects the throat to the middle ear. By opening this tube, air flows into the middle ear, restoring pressure balance. Conversely, as a diver approaches the surface, air in the middle ear expands (due to the decreasing pressure). The ear drum bulges outward as the middle ear expands like a balloon. By letting air escape the middle ear (again thru the eustachian tube) a diver can restore pressure balance during an ascent.

The same adjustments happen when we fly in airplanes. The higher a plane flies, the more pressure decreases, so once more the eustachian tube is put to work--the opposite happens when a plane descends.

Divers have to begin equalizing the pressure in their ears to the surrounding water pressure as soon as they start their descent from the waters's surface. One way to equalize is to hold your nose, as if you're avoiding a bad smell, close your mouth, and then very gently blow as if you're trying to blow your nose. It's very important not to blow too hard, however, because you can damage your ears if you do! When done correctly, you feel a little "pop", just like you feel sometimes when you're taking off in an airplane as your ears equalize to the changing pressure.

A diver has to repeat the equalizing procedure (often called "clearing") every so often as long as he or she continues to descend. Sometimes, if a diver descends too quickly, the surrounding water pressure will make clearing too difficult and the diver will have to actually go up a little bit, until he or she can successfully clear, and then continue downward. Descending to great depths has to be done gradually and carefully while clearing along the way. Usually it's well worth the trouble, though, with the amazing sights to be found underwater!

They don't. There is a limit to how deep divers can go because of the pressure. Divers can use an oxygen-helium mixture in the air that they breathe at moderate depths to prevent excess oxygen from being poisonous, but going to great depth is not currently possible.