Postojna Cave (Postojnska Jama) is the most visited cave in Europe and one of the most spectacular underground landscapes on the continent, a 24-kilometre karst cave system in southwestern Slovenia, carved by the Pivka River over millions of years, discovered in its modern form in 1818, and open to the public since 1819 (it has been a tourist attraction for over 200 years, the cave railway, installed in 1872, is the oldest underground railway in the world). The cave is famous for its stalactites, stalagmites, and the extraordinary formations that have earned it UNESCO World heritage status (as part of the Škocjan Caves), but its most famous inhabitant is the olm (Proteus anguinus), the “human fish,” a blind, translucent, cave-dwelling aquatic salamander that can live for 100 years and survive without food for a decade, one of the most remarkable creatures on Earth.

Quick Facts: Postojna Cave, Slovenia

• The experience: A 90-minute guided tour through 5 km of the cave, beginning with a 3.7-km train ride (the same open carriages have rattled through the cave for over 140 years, the sensation of speeding through the darkness, the rock formations flashing past, is one of the great underground experiences in Europe), followed by a 1.5-km walking tour through the most spectacular chambers, the Great Mountain (a 16-metre-high stalagmite), the Spaghetti Hall (the ceiling covered with thousands of thin, white stalactites, the most famous sight in the cave), the Brilliant (a 5-metre pure white stalagmite, the symbol of Postojna Cave), and the Concert Hall (the largest chamber, with perfect acoustics, concerts are held here, and the capacity is 10,000 people). The temperature in the cave is a constant 8–10°C, bring a jacket. The path is well-maintained and accessible for all ages and fitness levels
• The Vivarium: At the end of the tour, an aquarium exhibiting the olm (the “human fish”). The olm is extraordinary, a pale, blind amphibian with external gills and tiny limbs, adapted to complete darkness, found only in the karst caves of the Dinaric Alps. Postojna’s olms are famous, in 2016, an olm in the Vivarium laid eggs, and the hatching of the “baby dragons” (as the Slovenian press called them, with justification, the olms were once believed to be the offspring of a cave-dwelling dragon) was a national event. The Vivarium is included in the cave entry
• Best time to visit: Year-round (the cave is always 8–10°C, regardless of the season outside). The Living Nativity in December (a Christmas pageant performed by actors in the cave’s chambers, illuminated by thousands of candles) is a magical experience, book months ahead. Summer is the busiest, book online in advance to skip the queue. The cave is open every day of the year except Christmas Day
• Practical information: Postojna is ~50 km south of Ljubljana (~45 min by car or 1 hour by train). Entry: ~€26 (adult). Book online for a small discount and a guaranteed time slot. The Predjama Castle (9 km from Postojna, a medieval castle built into the mouth of a cave, one of the most dramatically situated castles in Europe. Entry: ~€14. Combined ticket with Postojna Cave: ~€35) is the essential pairing. Allow a full day for both