Deep diving

In deep diving, divers use a breathing apparatus (a supply of air carried on their backs). Or they may be supplied with breathing gas through a hose from above the water. This allows them to dive to some of the deeper parts of the ocean. Commercial divers may use different gas mixtures (Helium-Oxygen for example). They may also use armoured diving suits pressurized to one atmosphere. A deep dive is considered anything below a depth of about . This is still not very deep compared with the deepest parts of the ocean. Divers sometimes stay underwater for long periods of time. Sometimes they do work on oil wells and pipelines. Sometimes they explore sunken ships to see if they can retrieve items from the ship or help to bring the ship up to the surface. 
One of the dangers of deep diving is decompression sickness. That can happen if divers rise to the surface of the ocean too quickly. For that reason recreational divers need to be certified to show they have training to dive to a specified depth range.