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An underground lake or a subterranean lake is a lake under the surface of the Earth. Such a lake may be associated with caves, aquifers, or springs. They are typically very low in salinity.
The largest non-subglacial underground lake in the world is Dragon's Breath Cave, Namibia, with an area of almost 2 hectares (5 acres) and the second largest is Craighead Caverns in Tennessee, United States of America.
Media related to Underground lakes at Wikimedia Commons