Decompression is where you de-compress or return to the surface or shallower water. Under pressure, gasses dissolve into the blood. Sometimes decompressing quickly can mean that these gasses cannot come out of your lungs easily and the result on your body is similar to opening a bottle of carbonated drink. These bubbles are very dangerous and can cause decompression sickness. also known as the bends or caisson's disease. This can be avoided by not going very deep or staying down too long and returning to the surface slowly.