Subterranean River, Novosibirsk, Russia

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. Reversing this process is known as daylighting a stream and is a visible form of river restoration. One successful example is the Cheonggye Stream in the centre of Seoul.