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An underground lake or subterranean lake is a lake underneath the surface of the Earth. Most naturally occurring underground lakes are found in areas of karst topography, where limestone or other soluble rock has been weathering away, leaving a cave where water can flow and accumulate.
Natural underground lakes are an uncommon hydrogeological feature. More often, groundwater gathers in formations such as or springs.
The largest underground lake in the world is in Dragon's Breath Cave in Namibia, with an area of almost ; the second largest is The Lost Sea, located inside Craighead Caverns in Tennessee, United States, with an area of
The rarity of naturally occurring underground lakes can be attributed to the way water behaves underground. Below the surface of the Earth, the amount of pressure exerted on groundwater increases, causing it to be absorbed into the soil. The boundary at which there is sufficient sub-terranean pressure to completely soil saturation the ground with water is called the water table. The area above the water table is called the "unsaturated zone," while the area below it is called the "saturated zone". In the saturated zone, pressure becomes the primary force driving the flow of water. Lakes form primarily under the force of gravity – water is pulled down to the lowest point in an area, and gathers into a lake. Any water below the water table will be under pressure, and so does not form a lake; instead, it forms an aquifer.
Naturally occurring underground lakes can form in karst areas, where the weathering of soluble rocks leaves behind caverns and other openings in the earth. Surface water can find its way underground through these openings and pool up in larger caverns to form lakes.
Underground lakes can also be formed by human processes, such as the flooding of Mining. Two examples are the lakes found in the slate mines at Blaenau Ffestiniog, such as Croesor quarry, and a lake in the Hallein Salt Mine in Austria.
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