Saturation diving is a diving technique and associated technologies that allow divers to remain a great depth for long periods of time without suffering decompression sickness or having to spend an unacceptably high proportion of their working time doing decompression stops.
Commonly, saturation diving allows professional divers to live and work at depths greater than 50 metres / 165 feet for days or weeks at a time. While underwater, they live and work in a large "diving habitat" or a smaller diving bell. These are pressure vessels at ambient pressure with a hatch at the bottom allowing access to and from the water. They may also live in a pressurized habitat on the support ship, and be transported to and from there via airtight connectors. It often takes many days for the divers to decompress in a decompression chamber at the end of the dive.
"Saturation" refers to the fact that the diver's tissues have reached the maximum partial pressure of gas possible for that depth due to the diver being exposed to breathing gas at that pressure for prolonged periods. The significance of this is that once the tissues become saturated, the time to ascend from depth, to decompress safely, will not increase with further exposure.
The divers often use surface supplied diving equipment, or even SCUBA, using deep diving breathing gases stored in large diving cylinders on the "habitat" or diving bell. These gas supplies are delivered by crane from a diving support vessel.
Saturation diving (or more precisely, long term exposure to high pressure) is known to cause osteonecrosis , although it is not yet known if all divers are affected or only especially sensitive ones. The joints are most vulnerable to osteonecrosis. The connection between high pressure exposure and osteonecrosis is not fully understood.
Increased use of underwater ROVs and AUVs for routine or planned tasks means that saturation dives are becoming less common.
For saturation diving in fiction, see The Abyss.