Postojna Cave is the crown jewel of the Slovenian karst, a 24-kilometer subterranean wonderland carved over millions of years by the Pivka River. The visit begins aboard a unique electric cave train that whisks you 3.7 kilometers deep into the mountain, past towering stalagmites and shimmering curtain formations. From there, a guided walking tour reveals cathedral-sized chambers adorned with delicate spaghetti stalactites, massive columns, and the famous Brilliant — a five-meter-tall pure white stalagmite that has become the symbol of the cave. Keep your eyes peeled for the olm, a rare blind cave salamander nicknamed the “baby dragon” that lives in the underground pools.

Pro tip: Combine your visit with nearby Predjama Castle (€17.90 combo ticket), a jaw-dropping Renaissance fortress literally built into the mouth of a cave just 10 minutes away.