Postojna (Adelsberg, Postumia Grotte) is a town and a municipality in the province of Carniola, 35 kilometers (22 miles) from Trieste, in southwestern Slovenia. Population 14,581 (2002). The area is known to have been populated since the paleolithic era due to the finding of a cave settlement near the town of Postojna, called "Betalov spodmol".
Near the town is a major tourist attraction: the 20-km long cave system called Postojna Cave. The cave was first described in the 17th century by Janez Vajkard Valvasor, and a new area of the cave was discovered in 1818 by Luka Čeč. Electric lighting was added in 1872, enhancing the cave system's popularity. Approximately 9 kilometres from Postojna is Predjama Castle, set within the mouth of a cave.
Species unique to the Postojna cave system include the olm (Proteus anguinus), in Slovene literally known as the "human fish" (because of the color of its skin) and Leptodirus hochenwartii, a blind cave beetle.