changes for the workers and prevent excessive inflow of mud or water at the base of the caisson. When the caisson hits bedrock, the sandhogs exit through the airlock and fill the box with concrete, forming a solid foundation pier. A pneumatic (compressed-air) caisson has the advantage of providing dry working conditions, which is better for placing concrete. It is also well suited for foundations for which other methods might cause settlement of adjacent structures. Construction workers who leave the pressurized environment of the caisson must decompress at a rate that allows symptom-free release of inert gases dissolved in the body tissues if they are to avoid decompression sickness, a condition first identified in caisson workers, and originally named "caisson disease" in recognition of the occupational hazard. Construction of the Brooklyn Bridge, which was built with the help of pressurised caissons, resulted in numerous workers being either killed or permanently injured by caisson disease during its construction. Barotrauma of the ears, sinus cavities and lungs and dysbaric osteonecrosis are other risks. Other uses Caissons have also been used in the installation of hydraulic elevators where a single-stage ram is installed below the ground level. Caissons, codenamed Phoenix, were an integral part of the Mulberry harbours used during the World War II Allied invasion of Normandy. Other meanings Boat lift caissons: The word caisson is also used as a synonym for the moving trough part of caisson locks, canal lifts and inclines in which boats and ships rest while being lifted from one canal elevation to another; the water is retained on the inside of the caisson, or excluded from the caisson, according to the respective operating principle. Structural caissons: Caisson is also sometimes used as a colloquial term for a reinforced concrete structure formed by pouring into a hollow cylindrical form, typically by placing a caisson form below grade in an open excavation and pouring once backfill is complete, or by drilling at grade, although this can be problematic with deep caissons, as unsupported excavations can collapse before the caisson form can be inserted. In this manner, the earth placed around the empty caisson form provides stability and strength, allowing concrete to be poured with fewer complications and with less risk of a form blowout. While, technically, only the form itself is actually a caisson, it is not uncommon for any below-grade cast concrete pillar to be referred to as,