Caissons are open bottomed boxes or cylindrical casings of various sizes that are progressively dug into soils.  Upon installation at their final service elevation, caissons most commonly serve as the base formwork for the bottom-most element of a concrete foundation (such as a pier).  They can be sealed at the top and pressurized with air to work underwater or below the water table, if necessary.  Some sealed caissons contain an airlock which would allow personnel to enter the caisson for inspection or hand digging around obstructions encountered during placement.  Within an enclosed caisson, the open bottom face maintains air pressure by sealing itself against the soils that are gradually being removed from within the caisson’s side walls.  One of the most famous early uses of pressurized air caissons was during the construction of the Brooklyn Bridge.  Many workers returning from work in the caissons developed decompression sickness, also known as “the bends”, a newly discovered phenomenon at that time.