caisson /ˈkāˌsän, , ˈkeɪˌsɑn 
, ˈkās(ə)n, ˈkeɪs(ə)n / ▸ 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 New American Dictionary