Exposure of rock salt (halitite) in the Asse II mine, southern Lower Saxony, Germany. Stratigraphically it is either the rock salt of the Leine Formation (Na3) or the rock salt of the Staßfurt Formation (Na2), Zechstein Group, Upper Permian (see also talk page). The round opening (diameter about 40 cm) in the left half of the picture is a test well for heat conduction tests. The excellent heat conductivity of rock salt makes it an appropriate host rock for the permanent repository of radioactive waste.