Underwater diving
Scuba diving is done with metal cylinders carried by the diver to hold the breathing air. In open-circuit scuba the air comes out of the mouthpiece when the diver breathes in, and goes into the water when the diver breathes out. When a rebreather is used, some of the same air is breathed many times, and the machine puts oxygen into the air to make up for the oxygen that is used up, and takes out the carbon dioxide by using a chemical to absorb it. A rebreather lets the diver use less air, but it can go wrong in more different ways.