A caisson is a structure used in construction and engineering, particularly underwater projects such as bridge or dam construction. Its purpose is to provide a dry, protected environment for workers and construction materials. While water is kept out of the structure, air is allowed in, removing the need for diving equipment and the like. The caisson has been used in construction since the 1800s.

The caisson is generally a boxlike structure with an open bottom and, sometimes, an open top, as well. The structure keeps water out of the construction area while its open bottom allows workers to place foundations and piers in the seabed or riverbed. In shallow water, an open caisson is used; its open top allows light and air to enter from above the water line. For deep-water construction, a pneumatic caisson has a closed top; pressurized air is pumped in, and personnel enter and leave through an airlock. Either type has a sharply inclined lower edge, allowing the caisson to be deeply embedded in the ground so water cannot seep in.

A caisson must be firmly mounted on a stable foundation for maximum security and efficiency. Engineers prefer a layer of bedrock beneath the caisson. If bedrock is too far beneath the ground, they will sometimes rely on impacted mud or earth, or create an artificial foundation.

In pneumatic caissons, mud, rock and other unwanted materials are brought to the surface through the “muck tube,” which connects to the top of the caisson. Cranes on the surface move this material away from the construction site. The muck tube also helps to stabilize air pressure by releasing excess air. When construction within the caisson is completed, the structure can be repositioned to allow work to continue in another location.

Caissons were used in the construction of early U.S. suspension bridges such as the Eads Bridge in St. Louis, Missouri, and the Brooklyn Bridge in New York. Many workers, including Brooklyn Bridge chief engineer Washington Roebling, suffered from what then was called caisson disease; some died. Caisson disease was later discovered to be decompression sickness caused by the human body’s inability to adapt to rapid changes in external pressure. Modern caisson workers compensate for this by gradually returning to the surface, much as deep-sea divers do.

The term “caisson” also is used to refer to various other engineering and architectural structures. Caisson foundations can be embedded deep beneath the ground in non-aquatic construction. In shipping, caissons filled with water transport ships from one elevation to another in passageways such as canals. In architecture, the caisson is a boxlike structure used in the construction of domes or ceilings and sometimes called a coffer.

The military definition of a caisson also involves a box, one that carries ammunition during battle and the coffin during a military funeral. While different from the engineering caisson, all these devices share a similar shape. "Caisson" is a French word meaning "box".