Postojna Cave is a 24,120 m long karst cave system near Postojna, southwestern Slovenia. It is the second-longest cave system in the country after the Migovec System. The caves were carved by the Pivka River.
The cave was first described in the 17th century by the pioneer of study of karst phenomena, Johann Weikhard von Valvasor, although graffiti inside dated to 1213 indicates a much longer history of use. In 1818, when the cave was being prepared for a visit by Francis I, the first Emperor of the Austria-Hungary, a new area of the cave was discovered accidentally by Luka Čeč, a local man in charge of lighting lamps in the cave. In the 1850s, the Austrian-Czech geographer Adolf Schmidl published the first comprehensive scientific overview of the Postojna caves and the Pivka Basin, which became a standard reference point in the study of speleology.
The caves are also home to the endemic olm, the largest troglodytic amphibian in the world.