A subterranean or underground lake is a lake below the Earth's surface. Often found in caves or beneath glaciers, their source is commonly a spring, aquifer or glacier.
Discovered in 1986, the subterranean lake in Dragon's Breath Cave, Namibia, is the largest non-subglacial lake in the world, with a surface area of about five acres. The second largest is in the United States, in Craighead Caverns, in Sweetwater, Tennessee. The lake was discovered in 1905 by a 13-year-old boy while exploring the caverns. With a surface length of about 800 feet and a width of about 220 feet, it is a popular tourist attraction, often called The Lost Lake.
Subterranean lakes are usually very clear and low in salinity, mainly because they are protected from elements that stir up sediment, such as wind and runoff, and because the absence of sunlight means that there is little if any sunlight to power photosynthesis. They typically host few life forms other than microbes. In 2013, researchers in Antarctica analyzed the first sample of water ever retrieved from a sub-glacial lake. They found that the sample contained about 130,000 cells of bacteria and archaea per milliliter of water, a concentration commonly found in deep oceans.