Some 23% of the territory of Bulgaria is covered with carbonate rocks. The millennial physical effects and chemical reactions between the water and the rocks have produced diverse carvings on the surface and under it known as karst carvings. Caves, a magnificent world deep into the earth, are the most interesting. 124 of the 4500 caves in Bulgaria are natural sights. Duhlata at the village of Bosnek, Pernik district, is the longest cave in Bulgaria. The mapped passages in this gigantic maze of curving tunnels and halls chiseled by the water of the river Strouma in the bowels of Mount Vitosha have a length of 16,500 m. Listed as a natural sight in 1962.