Caisson

noun

1. An enclosure from which water can be expelled, in order to give access to underwater areas for engineering works etc.
2. The gate across the entrance to a dry dock.
3. A floating tank that can be submerged, attached to an underwater object and then pumped out to lift the object by buoyancy; a camel.
4. A two-wheeled, horse-drawn military vehicle used to carry ammunition (and a coffin at funerals).
5. A large box to hold ammunition.