If the diver is to work at fairly constant depths for periods which would require long periods for decompression, the diver may live in a special underwater habitat or a pressurised surface habitat called a saturation system. This type of diving is known as saturation diving. The same technique for supplying breathing gas as regular surface supplied diving is used, with the diving bell receiving breathing gas and other essential services from a diving support vessel on the surface. If diving at extreme depths, helium-based breathing gas mixtures are used to prevent nitrogen narcosis and oxygen toxicity which would otherwise occur due to the high ambient pressure. The diver is decompressed only once, at the end of the job, which saves time and reduces risk of decompression injury.