Ponor

A ponor is a natural surface opening that may be found in landscapes where the geology and the geomorphology is characterized by some kind of karst.

Whereas a sinkhole is a depression (doline) of surface topography with a pit or cavity directly underneath, a ponor is kind of a portal where a surface stream or lake flows either partially or completely underground into a karst groundwater system. Steady water erosion may have formed or enlarged the portal in (mainly limestone) rock, in a conglomerate, or in looser materials.