Subterranean river

[1] 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.

The longest subterranean river in the world is the Sistema Sac Actun cave system in Mexico.

[5][6] Some fish (colloquially known as cavefish) and other troglobite organisms are adapted to life in subterranean rivers and lakes.

The river Alph, running "Through caverns measureless to man / Down to a sunless sea" is central to the poem Kubla Khan, by Samuel Taylor Coleridge.

The characters in Jules Verne's Journey to the Center of the Earth encounter a subterranean river: "Hans was not mistaken," he said.

A subterranean river in the Cross Cave system of Slovenia . (Scale shown by people in photograph.)
The cave of source of the Buna can be entered by boat and dived through a cave system serving as an effluence of the Zalomka .
The Puerto Princesa cave can be entered by boat.
Devil's Throat Cave subterranean river from above
The Effra is one of the subterranean rivers of London . It empties into the Thames by Vauxhall Bridge , from which this photograph was taken.
In Dante 's Inferno , Charon ferries souls across the subterranean river Acheron .