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A ponor is a natural opening where surface water enters into underground passages; they may be found in karst landscapes where the geology and the geomorphology is typically dominated by porous limestone rock.
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.
- Dicken, Samuel N. (November 1935). "Kentucky Karst Landscapes". The Journal of Geology. 43 (7): 708–728. doi:10.1086/624363. JSTOR 30057941.
Since local terms such as "sink," "sink hole", "kettle", "bottom", etc., are vague and confusing the Slovene terminology ("ponor", "doline", etc.) is used for the karst forms.
- "The Karstography of the Dinaric Karst in Bosnia and Hercegovina" (.html). devonkarst.org.uk. Devon Karst. Retrieved 4 September 2016.
- "The Devon Karst Research Society" (.html). www.devonkarst.org.uk. Devon Karst. Retrieved 4 September 2016.