Sistema Sac Actun in the context of "Optymistychna Cave"

A subterranean river (also known as an underground 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.

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.