Caisson Cais"son, n. F., fr. caisse, case, chest. See 1st
Case.
1. (Mil.)
(a) A chest to hold ammunition.
(b) A four-wheeled carriage for conveying ammunition,
consisting of two parts, a body and a limber. In light
field batteries there is one caisson to each piece,
having two ammunition boxes on the body, and one on
the limber. --Farrow.
(c) A chest filled with explosive materials, to be laid in
the way of an enemy and exploded on his approach.

2.
(a) A water-tight box, of timber or iron within which work
is carried on in building foundations or structures
below the water level.
(b) A hollow floating box, usually of iron, which serves
to close the entrances of docks and basins.
(c) A structure, usually with an air chamber, placed
beneath a vessel to lift or float it.

3. (Arch.) A sunk panel of ceilings or soffits.

Pneumatic caisson (Engin.), 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.




I guess you meant the first meaning. Oops.