Postojna (pôˈstoinä) [key], Ger. Adelsberg, Ital. Postumia, town (1991 pop. 20,283), in Slovenia, on the Karst Plateau. A summer resort, it is famous as the site of Europe's largest stalactite caverns, which are traversed by a subterranean river. Formerly Austrian, the town passed to Italy in 1919; in 1947 it was ceded to Slovenia, then a constituent republic of Yugoslavia.