If a SCUBA diver stays under water, say at a depth of 100 feet (about 30 meters), for a certain period of time, some amount of nitrogen from the air will dissolve in the water in his or her 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 and/or make intermittent 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 (breathing 100-percent oxygen on the way to the chamber also helps). Then, the pressure is released slowly.
Decompression sickness, also known as the bends, is one danger of diving. Other dangers include nitrogen narcosis, oxygen toxicity and simple drowning (if you run out of air before making it back to the surface). If the diver decompresses properly, remains at "recreational depths" (less than 100 feet or so), and is careful about the air supply, the dangers can be largely eliminated. Proper training, good equipment and careful execution are the keys to safe diving.