The historical copper shale mine excavations on the south-eastern flank of Harz Mountains have cut into numerous large caves in gypsum and anhydrite. These caves are known as 'Schlotten' (pl., sg. Schlotte). The word is derived from the Early New High German meaning internal hollow formations allowing the drainage of water and already finds mention in 16th century literature.

However, these quite spectacular gypsum caves have never aroused the interest of the wider public. Discovered through mining, they have always been only accessible via pit shafts and galleries and invariably considered to be part of the mine. But in a scientific sense they are deep phreatic and hypogene caves in a parent rock of anhydrite or gypsum, in their natural state filled with water and without an entrance. They are unique geological outcrops in Zechstein (upper Permian), large karst caves of rare character and particular beauty as well as cultural witnesses to historical mining.

The miners used the 'Schlotten' for a long period of time to drain water from the mines (until the 18th century) and for economical reasons also to store unwanted spoil (until the 19th century). As the mine workings reached deeper levels, subsidence and flooding became more common and the intensity of the karst dissolution process increased. Problems of catastrophic proportions due to mine flooding were encountered in 1892 near Eisleben and in 1988 near Sangerhausen. The hydrological problems that confronted the copper shale mine excavations in the south-eastern Harz region are of geogenic origin. The exploitable seams, which on average slope between 3° and 8°, are covered with a between 4 and 7 meter thick layer of limestone ('Zechstein') with the characteristics of a karst aquifer. Above this a 60 m thick layer of anhydrite or gypsum is found, in which the 'Schlotten' are formed, notably on geological faults.

The relevance of the 'Schlotten' as a natural phenomenon was first appreciated in depth by Johann Carl Freiesleben (1774 - 1846). He described them scientifically in 1809 and campaigned emphatically for their preservation. With regard to this, the 'Wimmelburger Schlotten' near Eisleben were surveyed and geologically mapped by Anton Erdmann (1782 - 1848). The plan and side elevation of the cave survey were reproduced in copperplate and are considered to be the oldest publicized depiction of a gypsum cave in Germany. From the mid 1970's the 'Schlotten' became a subject of speleological research for a short period of time. The abandoned projects have only recently been re-established. Two of the 'Schlotten' are accessible via the Mining Museum Wettelrode: the 'Segen-Gottes-Schlotte' and the 'Elisabethschaechter Schlotte' near Sangerhausen. The 'Wimmelburger Schlotten' near Eisleben are the largest gypsum caves in Germany and to a certain extent accessible for research.