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The crystal cave is a chasm in the Upper Devonian limestone. Large parts of the walls are covered with countless calcite crystals and pearl sinter. In this form, the crystal jewelry on the walls is considered unique in Germany. With a length of about 170 m, a width of up to 26 m and a height of up to 30 m, the cave is the largest natural underground single cavity that can be visited in Germany. It is one of the highlights along the Lahn.