Postojna Cave is a 20,570 m long Karst cave system near Postojna, Slovenia. It is one of Slovenian top tourism sites. The caves were created by the Pivka River. The cave was first described in the 17th century by Johann Weichard Valvasor, and a new area of the cave was discovered accidentally in 1818 by local Luka Čeč, when he was preparing the hitherto known part of the cave for a visit by Francis I, the first Emperor of Austria. In 1819, the caves were opened to the public, and Čeč went on to become the first official tourist guide for the caves. 5.3 km of the caves are open to the public, the longest publicly accessible depth of any cave system in the world.
Hvala puno Tina! :)