When underwater a diver is under greater pressure then when at the surface of the water and the deeper you go down the higher the pressure. A diver is also inhaling pure pressurized oxygen so when they begin to go up toward the surface of the water the pressure on the diver is lessened which causes the gases in the blood stream to expand, most particularly nitrogen gas, due to the lower amounts of experienced pressure and the higher concentration of oxygen in the blood stream. Now if the diver surfaces to rapidly the expanding gas can travel to the brain damaging it and/or rupturing blood vessels which can cause hemorrhaging which in turn can cause death. The situation where a diver surfaces too quickly and the expanded gas damages their brain is called The Bends.
