cais·son 
(kāsŏn′, -sən)
(kāsŏn′, -sən)n.
1. A watertight structure within which construction work is carried on under water.
2. See camel.
3. A large box open at the top and one side, designed to fit against the side of a ship and used to repair damaged hulls under water. Also called cofferdam.
4. A floating structure used to close off the entrance to a dock or canal lock.
5.
a. A horse-drawn vehicle, usually two-wheeled, used to carry artillery ammunition and coffins at military funerals.
b. A large box used to hold ammunition.
[French, from Old French, large box, alteration (influenced by caisse, chest) of casson, from Italian cassone, augmentative of cassa, box, from Latin capsa.]
caissonescorting the coffin of President Ronald Reagan to the US Capitol Rotunda in 2004