In engineering, a type of foundation most commonly used underwater for a bridge, but sometimes used in building construction. It is a large hollow structure that is sunk down through the earth by workers excavating from inside it; ultimately it becomes a permanent part of the pier. There are three types: the open caisson, open at both top and bottom; the box caisson, closed at the bottom; and the pneumatic caisson, with an airtight chamber to accommodate submerged workers. Caisson columns, typically 2 ft (0.6 m) or more in diameter, may be used as an alternative to bearing piles. A round hole is dug or bored to a stable layer of earth and temporarily supported by a steel shell, then filled with concrete poured around a cage of reinforcing bars.