noun (n)

• large watertight chamber used for construction under water(noun.artifact)

  Synonym:
  caisson, pneumatic caisson
  

  source: wordnet30
• A water- tight inclosure, as of piles packed with clay, from which the water is pumped to expose the bottom (of a river, etc.) and permit the laying of foundations, building of piers, etc.(noun)

  source: webster1913