caisson

definition: large watertight chamber used for construction under water
example:
speech part: noun
synonyms: pneumatic caisson cofferdam
definition: a chest to hold ammunition
example:
speech part: noun
synonyms: ammunition chest
definition: a two-wheeled military vehicle carrying artillery ammunition
example:
speech part: noun
synonyms:
definition: an ornamental sunken panel in a ceiling or dome
example:
speech part: noun
synonyms: coffer lacuna

How caisson is being used?