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Postoina cave, Slovenia 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.