A closed or dry bell is a pressure vessel for human occupation which is lowered into the sea to the workplace, equalised in pressure to the environment, and opened to allow the divers in and out. These functional requirements dictate the structure and arrangement. The internal pressure requires a strong structure, and a sphere or spherical ended cylinder is most efficient for this purpose. When the bell is underwater, it must be possible for the occupants to get in or out without flooding the entire interior. This requires a pressure hatch at the bottom. The requirement that the bell retains its internal pressure when the external pressure is lowered dictates that the hatch opens inward, so that internal pressure will hold it closed.