Closed caissons are large, watertight structures used in construction, particularly for building foundations in water or soft soil. They are typically made of reinforced concrete and are designed to be sunk into the ground or seabed. The caisson is closed at the bottom, allowing workers to excavate soil from inside while keeping water out. Once the caisson is in place and the excavation is complete, it is filled with concrete to create a stable base for structures like bridges or piers. This method is effective for projects near bodies of water, as it provides a solid foundation while minimizing environmental disruption.