What is Underwater diving?
[[File:No48p173 sauvetage sebastopol-e03-scaphandre plongeur.jpg|thumb|Hard-hat diver working to raise sunken ships]] '''Underwater diving''' is going underwater to do things. There are different reasons to do this: *'''[[Recreational diving]]''' is done for fun. It is sometimes called sport diving. It can be done using air from cylinders carried by the diver. This is called [[scuba diving]]. It can also be done for shorter dives by holding your breath. This is called freediving or skindiving. *'''[[Military diving]]''' is diving for a military purpose. *'''[[Commercial diving]]''' is diving to do underwater work. This is done as part of a project where the work must be done under water. *'''[[Scientific diving]]''' is diving done to find things out for scientific purposes. It may be part of the job for a scientist, or done by volunteer for a [[citizen science]] project. *'''[[Public safety diving]]''' is diving done by a public service organisation like the police of a fire department to rescue people or to find and bring back bodies or evidence. There are also different ways to dive: *'''[[Surface supplied diving]]''' gets air for breathing through a hose from a pump above the surface of the water. *'''[[Scuba diving]]''' is done with metal [[cylinder]]s 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. *'''[[Saturation diving]]''' is diving where the divers stay in pressurised rooms between dives and go to the underwater workplace and return in a sealed [[diving bell]], so they stay at the same pressure for days or weeks at a time before the pressure is reduced t