caisson

noun

1. A (permanent) enclosure from which water can be expelled, in order to give access to underwater areas for engineering works etc.

   “During the construction of the viaduct, the sinking of the caissons in the river bed caused much difficulty to the engineer and contractors, as a bed of running sand was encountered; in consequence, the expenditure for these foundations proved almost prohibitive.”

   “Caissons were enclosed dry chambers built on river beds to facilitate the construction of bridge piers.”

2. A structure with similar function that is temporary.
3. A coffer (sunken panel).
4. The gate across the entrance to a drydock.
5. A floating tank that can be submerged, attached to an underwater object and then pumped out to lift the object by buoyancy.

   “Near-synonym: pontoon”

6. A two-wheeled, horse-drawn military vehicle used to carry ammunition (and a coffin at funerals).

   “Near-synonym: limber”

   “Over hill, over dale / As we hit the dusty trail, / And those caissons go rolling along.”

7. A large box to hold ammunition.
8. A chest filled with explosive materials, used like a mine.