1) Karst formation
Found on
https://www.crosswordclues.com/clue/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
A cave or a sink hole in the low point of a polje, down which a river or stream disappears.
Found on
https://www.encyclo.co.uk/local/23001
No exact match found.
