Shown above is a feature called a tumulus in the Harz region of Germany, near Walkenried . Tumuli are rather rare and are usually only found in karst areas where gypsum occurs. The genesis of these hollow, subcircular domes can be explained by hydration of minerals such as anhydrite . Early inhabitants of the Harz region guessed that tumuli were the home of dwarfs. Note, prehistoric burial mounds are a different kind of tumuli . Photo taken on May 12, 2013.