A hollow, usually inverted vessel, such as one used for diving deep in a body of water. It is open on the bottom and supplied with air under pressure. During the Monitor 2001 Expedition, navy divers utilized a 12-person, two-chamber saturation system with a two-person closed diving bell. The system can operate as deep as 1,500 ft—considerably deeper than the Monitor, which rests on the sea floor at a depth of 235 ft. Saturation systems are often used in deep-water situations (below 200 ft) in order to reduce the time lost to decompression during the slow ascent to the surface required for preventing decompression sickness.