Aponor is a natural opening where surface water enters into underground passages; they may be found inkarst landscapes where the geology and the geomorphology is typically dominated by porouslimestone rock. Ponors can drain stream or lake water continuously or can at times work assprings, similar toestavelles.[1] Morphologically, ponors come in forms of large pits andcaves, largefissures and caverns, networks of smaller cracks, and sedimentary,alluvial drains.[1]
The name for the karst formationponor comes fromSerbo-Croatian andSlovene.[2] It derives from theproto-Slavic word*nora, meaningpit,hole.[3]
Several places in southeast Europe (Bulgaria, Bosnia and Herzegovina, Croatia, Czech Republic, Hungary, Romania, Montenegro, Serbia, and Slovenia)bear the namePonor due to associated karst openings.
Whereas asinkhole (doline) is a depression 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 karstgroundwater system.[1]
Steady water erosion may have formed or enlarged the portal in (mainlylimestone) rock, in aconglomerate, or in looser materials. Karst terrains are known for surface water losses through smallponors and its resurgence after having traveled through vast underground systems.[1]
Ponors are found worldwide, but only in karst regions. The entireAdriatic watershed withinBosnia and Herzegovina sits on Dinaric karst, with numerous explored and probably many more unexplored ponors and underground flows.[4][5] There are significant geological ponors in theCarpathian Mountains, theDinaric Alps, Greece, Turkey, and parts of the southern United States.[citation needed]
Reservoirs in karst are prone to losses due to leakage through ponors. The construction ofdams to capture water in karst terrains may pose a greatfinancial risk despite initial investigations and thorough sealing treatments.[6] It wasn't until the twentieth century that the first dams in karst were built, some of which famously failed.[6]
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.