In engineering, a caisson is a retaining, watertight structure used, for example, to work on the foundations of a bridge pier, or for the repair of ships. These are constructed so that the water can be pumped out so the working environment is dry.
Shallow caissons may be open to the air, while deep caissons to penetrate soft mud may be sealed at the top and filled with compressed air to keep water and mud out at depth.
Caisson disease is so named since it appeared in construction workers when they left the caisson and had rapid decompression. It is caused by the same processes as decompression sickness in divers. The Brooklyn Bridge was constructed with the help of caissons, and several workers died of caisson sickness.