Postojna Cave (Slovene: Postojnska jama; German: Adelsberger Grotte; Italian: Grotte di Postumia) is a 20,570 m long Karst cave system near Postojna, Slovenia. It is the second longest cave system in the country (following the Migovec Cave System) as well as one of its top tourism sites. The caves were created by the Pivka River.
The cave was first described in the 17th century by Johann Weichard Valvasor (Slovene: Janez Vajkard Valvasor), and a new area of the cave was discovered accidentally in 1818 by local Luka Čeč, when he was preparing the hitherto known parts 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. Electric lighting was added in 1884, preceding even Ljubljana, the capital of Carniola, the Austro-Hungarian province the cave was part of at the time, and further enhancing the cave system's popularity.
In 1872 rails were laid in the cave along with first cave train for tourists. At first, these were pushed along by the guides themselves, later at the beginning of the 20th century a gas locomotive was introduced. After 1945, the gas locomotive was replaced by an electric one. 5.3 km of the caves are open to the public, the longest publicly accessible depth of any cave system in the World.
The caves are also home to the endemic olm, the largest trogloditic amphibian in the world. The tour through the caves includes an aquarium with some olms in it.