Synonyms for Caisson (same or very similar meaning)
WordNet sense 1 (an ornamental sunken panel in a ceiling or dome):
coffer, lacunaWordNet sense 2 (a chest to hold ammunition):
ammunition chestWordNet sense 3 (large watertight chamber used for construction under water):
cofferdam, pneumatic caissonFrom the ODE community, based on WordNetadd/edit