What is rebreather diving?

Rebreather diving is a form of technical diving in which the diver is using a closed circuit breathing set.  In other words, when the diver breathes out, instead of the exhaled gas going into the water, it is kept in the system and is recycled so that the unused oxygen can be used again.  This is based on the premise that when a diver breathes in, not all of the oxygen in the gas is absorbed by the lungs.  Instead of wasting it, it is better and more economical to recycle the gas and reuse the oxygen.  The rebreather is an apparatus that takes the exhaled breath, removes the carbon dioxide, and adds extra oxygen so that the diver can breathe that air again.