Saturation Diving is deep-sea commercial diving in which the diver's bloodstream is saturated with a mixture of helium and oxygen equalised to the same pressure of the surrounding water, so that the decompression time following the dive is independent of the duration of the dive itself. This enables the divers to live in a saturation diving chamber located within a Diving Support Vessel. The following videos give a very good explanation of how Saturation Diving took place then and now, and the dangers involved and associated with commercial saturation diving.  

During the mid 1970's commercial diving was a necessity to bring oil ashore once it had been discovered in the North Sea. Pipelines would run from rigs to shore and divers were required to assemble both the subsea installations and pipelines themselves. It was a requirement for commercial divers to live at the same pressure as that in which they were working. Saturation diving was seen as the best way to safely enable them to work at such depths for long periods of time. Otherwise divers would need to re-compress after each dive, a long drawn-out process requiring many divers to do the work.