The 
bends (properly called 
decompression sickness) is a condition experienced by 
divers who ascend too quickly.  When air under high pressure is breathed for any length of time, 
nitrogen dissolves in the body fluids: release of the outside pressure 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. 
Because helium has a far lower solubility in water, deep-sea divers nowadays breathe heliox (helium/oxygen mix).
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.
Other ill effects experienced by divers are nitrogen narcosis and oxygen toxicity[?].
The Bends is a 1995 album by British band Radiohead.
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