Postojna Cave in western Slovenia is a 20-kilometre (12-mile) labyrinth of subterranean passages, filled with fantastical stalagmites, stalactites and other rock formations. It is considered by experts to be one of the finest examples of karst landscape, where limestone rock has been heavily eroded to form underground streams, a phenomenon that has created several other caves in the area, the temperature inside the caves is a constant 8° C.