Scuba stands for self-contained underwater breathing apparatus. When using scuba equipment,you breathe from a tank that holds highly compressed air.

When you breathe from a scuba tank, the air coming out of the tank has the same pressure as the pressure that the water has, if not it won't come out of the tank. Therefore, when scuba diving, the air in your lungs at a 33-foot depth has twice the pressure of air on land.

When high-pressure gases in the air come in contact with water, they dissolve into the water.

If a scuba diver stays under water, for a certain period of time, some amount of nitrogen from the air will dissolve in the water in the body. If the diver were to swim quickly to the surface, it is just like uncorking a bottle of soda, the gas is released. This can cause a very painful condition, and it is sometimes fatal.

To avoid the effects of quick decompression, the diver must rise slowly or make various stops on the way up (called "decompression stops") so that the gas can come out of solution slowly. If the diver does rise too fast, the only cure is to enter a pressurized chamber in which the air pressure matches the pressure at depth. Then, the pressure is released slowly.

Decompression sickness(DCS), also known as the bends, is one danger of diving. If the diver decompresses properly, and is careful about the air supply, the dangers can be avoided. Proper training, good equipment and careful execution are the keys to safe diving.

The "bends" is another name for Caisson disease. The word "caisson" means a chest for the transportation of explosives or ammunition, but the word came to mean a large water-tight case or chest used in laying foundations of bridges in deep water.