caisson

 
Pronunciation: /ˈkeɪs(ə)n, kəˈsuːn/

noun

  • 1a large watertight chamber, open at the bottom, from which the water is kept out by air pressure and in which construction work may be carried out under water.
  • a floating vessel or watertight structure used as a gate across the entrance of a dry dock or basin.
  • 2 historical a chest or wagon for holding or conveying ammunition.

Origin:

late 17th century: from French, literally 'large chest', from Italian cassone, the spelling having been altered in French by association with caisse 'case'