Caissons are large, watertight structures used in construction, particularly for building foundations underwater or in areas with high water tables. They are typically made of reinforced concrete or steel and can be sunk into the ground to create a dry working environment. This allows workers to safely excavate and lay the foundation for structures like bridges and piers. There are two main types of caissons: open caissons, which are open at the top and allow water to enter, and closed caissons, which are sealed at the top and filled with air or other materials. Caissons are essential for ensuring stability and safety in underwater construction projects.