underground lake; subterranean lake
A lake located beneath the earth's surface, often in caves or deep geological formations.
A beautiful underground lake spread out deep inside the cave.
The water of the subterranean lake was surprisingly clear.
Means 'cave'; a 地底湖 is often found inside a 洞窟.
Compound of 地底 (chitei, 'underground depths') and 湖 (ko, 'lake').