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 complet...
Found on http://en.wikipedia.org/wiki/Ponor

Ponor

A cave or a sink hole in the low point of a polje, down which a river or stream disappears.
Found on http://www.bgs.ac.uk/discoveringGeology/glossary.html
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