noun
underground lake; subterranean lake
A lake located beneath the earth's surface, often in caves or deep geological formations.
その洞窟の奥には美しい地底湖が広がっていた。
A beautiful underground lake spread out deep inside the cave.
地底湖の水は驚くほど澄んでいた。
The water of the subterranean lake was surprisingly clear.
Compound of 地底 (chitei, 'underground depths') and 湖 (ko, 'lake').