Caisson

N
1-a bottomless watertight chamber filled with compressed air, used as a base from which construction work is carried out underwater
2-a hollow structure attached to a sunken object, for example, a wrecked ship, then pumped full of air until it acts as a float, raising the object to the surface
3-a floating watertight structure used to keep water from entering a dry dock, canal lock, or basin
4-a large container for ammunition
5-a two-wheeled horse-drawn vehicle, formerly used to carry ammunition but now often used to carry coffins at state or military funerals
6-see coffer