A caisson is a large, watertight structure used in construction, particularly for underwater projects. It allows workers to safely excavate and build foundations below the water level. Caissons are often made of reinforced concrete or steel and can be open at the top or closed, depending on the project requirements. These structures are commonly used in the construction of bridges, piers, and tunnels. When building a bridge, for example, caissons provide a stable base for the bridge supports by preventing water from flooding the work area. This technique is essential for ensuring safety and structural integrity in challenging environments.