Decompression sickness (also known as the bends) is a condition experienced by anyone going from high pressure to lower pressure, and is typically experienced by divers who ascend too quickly from depths below 10 meters.
When air under high pressure is breathed for any length of time, nitrogen dissolves in the body fluids in higher than usual concentration. When moving to lower pressure, this causes the gas to come back out of solution, and form bubbles in the blood. The physiologist J.S. Haldane studied this problem in the early 20th century, eventually devising the method of staged, gradual decompression, whereby the pressure on the diver is released slowly enough that the nitrogen comes gradually out of solution without forming bubbles. Repeated cases of decompression sickness can lead to brittle bones, and severe cases can lead to death. Severe cases of decompression sickness require treatment by recompression in a hyperbaric chamber.
An alternative name is caisson disease; this name comes from the 19th century, when large engineering excavations (bridges, tunnels) required the work to be done in "caissons" under pressure to keep water from flooding the excavations. This was a major factor for laborers working on the Brooklyn Bridge, and incapacitated the project leader Washington Roebling.
Because helium has a far lower solubility in water, decompression sickness can be avoided by breathing a mixture of helium and oxygen (heliox) instead of air. This is often practised by deep-sea divers.
Other ill effects experienced by divers are nitrogen narcosis and oxygen toxicity.