Notranjska is also called Inner Carniola, in German Innerkrain. The administrative and economic center of the region is Postojna.
The central Notranjska Karst area is a heavily karstified triangle located around Postojna, Planina and Cernica. Notranjska has numerous important cave systems, dolines, swallow holes, seasonal lakes, and karst springs. There is a characteristic change between insoluble flysch lowlands and soluble limestone ridges. Rivers flow on the surface where the rock is flysch, and leave the surface, as soon as they reach the limestone. They disappear underground, just to reappear a few kilometers away behind the karst ridge.
The most famous cave system is the underground bed of the Pivka river. This cave system is enormous, more than 24km long and with numerous entrances in the whole area. In former times, when the people knew nothing about the connection of the caves, they gave every entrance its own name. Several cave entrances where developed as independent show caves. But when the caves were connected underground, the resulting cave had numerous names.
This area has a long speleological history, the first known visit of the cave happened already in the 13th century. During the renaissance, in the 16th century, visitation and exploration of the cave boosted. The Postojna branch, better the upper level of Postojna Cave which is visited today, was discovered in 1818. The first light system was installed in 1818 using gas. In 1884 electric light was installed, one of the first caves in the world.
The reason is easy: the southern entrance of the cave system is really huge. The cave is big and rather easy to explore, you just walk in. And there is the history of the area. Slovenia belonged to the Austrian Hungarian Empire, an enormous multi cultural empire unique in history. So there was the possibility for famous geologists from all over Europe to visit and explore the cave.