Postojna cave is one of the most beautiful caves in the world. The parts of the Postojna Cave System as it is known today cover a total of 21 kilometres and include: Postojna Cave, the Otok Cave, the Pivka Cave, the Black Cave and Planina Cave. With the exception of the human fish, whose life was long shrouded in mystery and superstition, the subterranean fauna has not been the subject of study for very long. Subsequent research has shown that 84 species of specialised subterranean fauna live in the cave. The dry sections of the caves are for the most part open to visitors. One of the elements that make the caves so attractive to visitors is the cave formations or speleotherms (stalagmites, stalactites, flowstones, etc.) resulting from deposits of calcium carbonate. Their rate of growth is so slow as to be almost imperceptible. The cave environment is less subject to changes of temperature than the outside world. Throughout the year the temperature remains at approximately 10 degrees Celsius. In summer it is slightly higher because of heat from outside, while in winter it drops slightly. The humidity of the air is high. Visitors to the cave should dress appropriately and according to the season. Due to the sensitivity of the natural environment, visitors must follow the instructions of the guides.