The jaw-dropping Postojna Cave system, a series of caverns, halls and passages some 24km long and two million years old, was hollowed out by the Pivka River, which enters a subterranean tunnel near the cave’s entrance.

　　Visitors get to see 5km of the cave on 1½-hour tours; 3.2km of this is covered by a cool electric train. Postojna Cave has a constant temperature of 8°C to 10°C, with 95% humidity, so a warm jacket and decent shoes are advised.

　　The train takes you to the Great Mountain cavern, on a trip that's like entering the secret lair of a James Bond villain. From here a guide escorts you on foot through tunnels, halls, galleries and caverns in one of four to six languages (audioguides are available in many more tongues).

　　These are dry galleries, decorated with a vast array of stalactites shaped like needles, enormous icicles and even fragile spaghetti. The stalagmites take familiar shapes but there are also bizarre columns, pillars and translucent curtains that look like rashers of bacon.

　　From the Velika Gora cavern you continue across the Russian Bridge, built by prisoners of war in 1916, through the 500m-long Beautiful Caves that are filled with wonderful ribbon-shaped stalactites and stalagmites that are two million years old (it takes 30 years to produce 1mm of stalactite). The halls of the Beautiful Caves are the farthest point you’ll reach; from here a tunnel stretches to the Black Cave (Črna Jama) and Pivka Cave (these can also be visited on additional tours).

　　The tour continues south through the Winter Hall, past the 5m, snow-white Brilliant stalagmite (also sometimes called the Diamond) and the neighbouring baroque pillar, which have become symbols of the cave. You then enter the Concert Hall, which is the largest in the cave system and can accommodate 10,000 people for musical performances. In the week between Christmas and New Year, the Live Christmas Crib (Jaslice) – the Nativity performed by miming actors – also takes place in the cave. Visitors reboard the train by the Concert Hall and return to the entrance. The river continues its deep passage underground, carving out several series of caves, and emerges again as the Unica River.

　　Green felt capes can be hired at the entrance for €3.50. There are few steps to climb. Check the website for package deals, including various combination tickets that include Vivarium Proteus, the new Expo and the don't-miss Predjama Castle. Postojna is a less-strenuous option than Škocjan Caves.