Ponnur, India

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. Ponors can drain stream or lake water continuously or can at times work as springs, similar to estavelles. Morphologically, ponors come in forms of large pits and caves, large fissures and caverns, networks of smaller cracks, and sedimentary, alluvial drains.