noun
Refers to a natural underground hollow, often with rocky walls; somewhat literary or formal compared to 洞窟 (どうくつ).
その島には海に面した岩窟がいくつもある。
The island has several caves facing the sea.
探検隊は深い岩窟の中で古代の壁画を発見
The expedition team discovered ancient wall paintings inside a deep cavern.
洞窟 is the common word for 'cave'; 岩窟 emphasizes rocky walls and is more literary.
Compound of 岩 (rock) and 窟 (cavern). The exact historical derivation is uncertain, but the spelling is conventionally associated with a rocky cave.