In deep diving, divers use a breathing apparatus (a supply of air carried on their backs) or may be supplied with breathing gas through a pipe from above the water. This allows them to dive to some of the deeper parts of the ocean, but still not very deep compared with the deepest parts. They sometimes stay there 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.