Underground lake
An underground lake or subterranean lake is a lake under the surface of the Earth. Such lakes may be associated with caves, aquifers, or springs. They are typically very low in salinity. Underground lakes often form in abandoned quarries, but can also form naturally. In most cases, naturally-formed underwater caves are an artifact of Karst topography. The largest non-subglacial lake in the world is in Dragon's Breath Cave in Namibia, with an area of almost , and the second largest is in Craighead Caverns in Tennessee, United States.