A ponor is a natural opening that allows surface water to enter underground passages. These geological features are typically found in karst landscapes where the geomorphology and geology are dominated by porous limestone rock.
Ponors can function as continuous drains for streams or lakes, or they may occasionally act as springs, similar to estavelles. Morphologically, these openings manifest in various forms, including large pits, caves, fissures, caverns, networks of smaller cracks, and sedimentary or alluvial drains.