caisson /ˈkeɪsn 
, ˈkeɪsɒn , kəˈsuːn / ▸ noun 1 a 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’.
EnglishOxford Dictionary of English