The Ajdar valley is located in Afghanistan. A natural dam blocks the barren valley. Reputed to be a petrified dragon, the barrier is actually made of travertine.

An impressive rock dam about 260 feet (80 meters) high and 40 feet (12 meters) wide blocks this arid valley deep in the heart of Afghanistan. Ajdar is the Persian word for "dragon," and legend has it that the dam is really a petrified dragon that was slain by the son-in-law of the Prophet Mohammed after it demanded a daily tribute of one maiden, two camels, and a ration of fodder. Actually, the dam is a natural barrier anchored to an outcrop of limestone and and consisting of accumulations of travertine, which is made of the same substance (calcium carbonate) as limestone. Even in this dry area even the underground water is rising to the surface in springs. As it does so, it dissolves calcium carbonate from the limestone. Upon reaching the surface, the pressure decreases, and the calcium carbonate is redeposited as travertine. (Specially adapted plants living here also help with it's precipitation.) As long as the groundwater continues to flow upward in this dry region, the natural dam will continue to grow.