This is a river that runs wholly or partly beneath the ground surface one where the riverbed does not represent the surface of the Earth (rivers flowing in canyons are not classed as subterranean.
Subterranean rivers may be entirely natural, flowing through cave systems. In karst topography, rivers may disappear through sinkholes, continuing underground. In some cases, they may emerge into daylight further downstream.
Subterranean rivers can also be the result of covering over a river and/or diverting its flow into culverts, usually as part of urban development. (2) Reversing this process is known as day lighting a stream and is a visible form of river restoration. One successful example is the Cheonggye Stream in the centre of Seoul.
Examples of subterranean rivers also occur in mythology and literature.