A caisson is a watertight, boxlike structure or chamber used in underwater excavation and construction, notably bridge building. It allows people to work underwater and makes it easier to remove excavated material.
Box caissons are used where the existing bed is firm. They are open at the top, usually being built on land, floated into position, filled with concrete, and sunk.
Open caissons have neither top nor bottom, their bottom edge being sharp. They are placed on the bed and excavation inside them proceeds. The caisson sinks, and as it does so its walls are built up, the increased weight assisting the sinking. When a firm bed is reached, the caisson is filled with concrete.
Pneumatic caissons have no bottom, and are filled with air under pressure to prevent the entry of water. Workers may enter via an air lock and excavate from within the caisson, which on completion is filled with concrete.