Caisson /(?)/

Cais·son

Caisson

n.
  1. A chest to hold ammunition. (Mil.)
  2. A water-tight box, of timber or iron within which work is carried on in building foundations or structures below the water level.
  3. A sunk panel of ceilings or soffits. (Arch.)

Phrases & Compounds
Pneumatic caisson
a caisson, closed at the top but open at the bottom, and resting upon the ground under water. The pressure of air forced into the caisson keeps the water out. Men and materials are admitted to the interior through an air lock. See Lock.