caisson

noun8

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

   • Caissons were enclosed dry chambers built on river beds to facilitate the construction of bridge piers. — 2003
2. •
   A structure with similar function that is temporary.
3. •
   A two-wheeled, horse-drawn military vehicle used to carry ammunition (and a coffin at funerals).

   • Over hill, over dale / As we hit the dusty trail, / And those caissons go rolling along. — 1908
4. •
   A coffer (sunken panel).
5. •
   A floating tank that can be submerged, attached to an underwater object and then pumped out to lift the object by buoyancy.
6. •
   The gate across the entrance to a drydock.
7. •
   A large box to hold ammunition.
8. •
   A chest filled with explosive materials, used like a mine.