-What you can expect- Ride and underground cave train through the second-largest underground cave system in the world and tour the Predjama Castle on a day trip with private transportation from Ljubljana.Explore the mysterious Karst underground in Postojna Cave, unique for its cave train, that will take you to the heart of the underworld and home of proteus.Afterward visit the biggest cave castle in the world, Predjama Castle, one of the most spectacular medieval castles, full of legends and old stories.