a chest or wagon for carrying ammunition, esp for use by large guns and cannons
a watertight chamber, usu with a supply of compressed air, used to house people doing construction work under water
a float for raising a sunken vessel
a watertight structure used to keep water in or out of a dock, harbour, etc
[French caisson, augmentative of caisse box, via Old Provençal from Latin capsa: see case2]