The Mark I Deep Dive System (DDS) was built in 1968 to provide a surface habitat for saturation divers working at great depths. Divers are sealed into the habitat and compressed to the same depth at which they will be working. After their bodies have become saturated, the divers move from the surface to the working depth and return in the diving bell commonly called a Personal Transfer Capsule (PTC). By always maintaining pressure, work continues on a 24 hour schedule with a 4 person crew of divers working and resting alternately for 2 or 3 weeks before decompressing to surface pressure. The Deck Compression Chambers (DDC) are designed to maintain pressure equivalent to a seawater depth of 1000 feet, and they are also equipped with a medical lock for passing food, supplies, and small articles in and out.