cave; rock chamber
Refers to a natural cave or a chamber formed within rock. Often used in geographical or historical contexts.
その島には大きな岩室がある。
There is a large cave on that island.
Compound of 岩 (iwa, 'rock') and 室 (muro, 'room' or 'chamber'). The exact historical derivation is uncertain, but the spelling is conventionally associated with a cave or rock chamber.