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.
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). The word "ponor" itself comes from Slovene in which word ponor has the same meaning.
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 karst regions. The entire Adriatic watershed within Bosnia and Herzegovina sits on Dinaric karst, with numerous explored and probably many more unexplored ponors and underground flows.
There are also several places in southeast Europe (Bulgaria, Croatia, Czech Republic, Hungary, Romania, Montenegro, Slovenia) 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 United States.