These mountains are a unique karstic landscape, characterised by the phenomenon being present on the surface (eksokarst) and subsurface (endokarst). Surface phenomena include positive formations such as karst hills totalling ± 40,000 conical hills, negative formation of karstic valleys and karstic lake. The subsurface phenomenon includes karst caves consisting of no less than 119 caves with stalactites and stalagmites, and underground rivers. Because of the uniqueness of its ecosystem, the 1993 International Union of Speleology proposed the Karst Area of the Sewu Mountains to be classed as a natural world heritage site. In the caves are also found human fossils from the early stone age about 1.8 million years old, along with stone tools for hunting.[3] Sewu mountains were formed with the lifting of the seabed thousands of years ago. Limestone rocks are characteristic of these mountains. The limestone hills are reported to contain hundreds of caves. These are classified locally as vertical caves (known as luweng in Javanese) and horizontal caves. Jomblang cave(id) (Luweng Jomblang) and Grubug Cave (Luwung Grubug), located in the Semanu subdistrict in Gunungkidul, as well as other caves in the area, are well-known to local caver (speleological) groups. Some of the caves are quite long; Cerme cave, for example, has an entrance in Bantul Regency and stretches for quite a distance eastward into Panggang subdistrict in Gunung Kidul Regency.