A subterranean river is a river that runs wholly or partly beneath the ground surface – one where the riverbed does not represent the surface of the Earth. It is distinct from an aquifer, which may flow like a river but is contained within a permeable layer of rock or other unconsolidated materials. A river flowing below ground level in an open gorge is not classed as subterranean. Some fish (colloquially known as cavefish) and other troglobite organisms are adapted to life in subterranean rivers and lakes. Examples of subterranean rivers in mythology and literature.