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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.
Ponors are found worldwide, but only in some karst regions. There are several places in southeast Europe (Bulgaria, Croatia, Czech Republic, Hungary, Romania, Montenegro) with the name Ponor due to associated karst openings. There are significant geological ponors in the Carpathian Mountains, the Dinaric Alps, Greece, Turkey, and parts of the southern U.S.
The term ponor has become the international geological term for larger karst-induced surface water inlets. The word derives from the proto-Slavic nora 'pit, hole, abyss'.