River that runs wholly or partly beneath the ground surface
A subterranean river is a
river or
watercourse that runs wholly or partly beneath the
ground, 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.[1]
Some natural rivers may be entirely subterranean, collecting in and flowing through
cave systems. In
karst topography, rivers that originate above ground can disappear into
sinkholes, continuing underground until they reappear on the surface downstream, possibly having merged with other subterranean rivers. The longest subterranean river in the world is the
Sistema Sac Actun cave system in Mexico.[2]
Subterranean rivers can also be the result of covering over a river or diverting its flow into
culverts, usually as part of
urban development.[3] Reversing this process is known as
"daylighting" a watercourse and is a major form of visible river restoration. Successful examples include the
Cheonggyecheon in the centre of
Seoul.[4][5]
Some fish (colloquially known as
cavefish) and other
troglobite organisms are adapted to life in subterranean rivers and lakes.[6]
Examples of subterranean rivers
also occur in mythology and literature.
Natural examples
There are many natural examples of subterranean rivers. Among them:
In many cities there are natural streams which have been partially or entirely built over. Such man-made examples of subterranean
urban streams are too numerous to list, but notable examples include: