Caissons are large, watertight structures used in construction, particularly for building foundations underwater. They are typically made of reinforced concrete or steel and are designed to be sunk into the ground or seabed. Once in place, the caisson is filled with concrete to create a stable base for structures like bridges or piers. The use of caissons allows engineers to work in challenging environments, such as rivers or coastal areas, where traditional foundation methods may not be feasible. They help prevent water from entering the construction site, ensuring a dry and safe working area for construction crews.