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 weathered 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 aquifers or springs.
Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mineral-rich waters, such as chalybeate springs. Fresh water may encompass frozen and meltwater in ice sheets, ice caps, glaciers, snowfields and icebergs, natural precipitations such as rainfall, snowfall, hail/sleet and graupel, and surface runoffs that form inland bodies of water such as wetlands, ponds, lakes, rivers, streams, as well as groundwater contained in aquifers, subterranean rivers and lakes.
Water is critical to the survival of all living organisms. Many organisms can thrive on salt water, but the great majority of vascular plants and most insects, amphibians, reptiles, mammals and birds need fresh water to survive.
Cross Cave (Slovene: Križna jama, German: Kreuzberghöhle), also named Cold Cave under Cross Mountain (Mrzla jama pod Križno goro), is a cave in Slovenia's Lož Valley, in the area between the Lož Karst Field, Cerknica Karst Field, and Bloke Plateau. The cave is named after nearby Holy Cross Church in Podlož. The cave is particularly noted for its chain of subterranean lakes. Extremely slow-growing calcareous formations (up to 0.1 mm per year) and their fragility are the main obstacle to large-scale tourism in the cave and limit daily tourist visits to the flooded part of the cave to four people. As a result, the Cross Cave is among the best-preserved caves, opened to the public in Slovenia. The cave was prepared for visits in the 1950s by the Lož Valley Tourist Association. It was later managed by the Ljubljana Cave Research Society. Since the 1990s, it has been cared for by the Friends of Cross Cave Association (Društvo ljubiteljev Križne jame).With 45 species of organisms, some not discovered until 2000, Cross Cave is the fourth-largest cave ecosystem in the world in terms of biodiversity. The cave was first documented in 1832, but the part of the cave that includes lakes and stream passages was first explored by Slovene cavers in 1926.
On January 6, 2024, five people were trapped in the cave due to high water levels caused by heavy rainfall. They were rescued on January 8, 2024.