Postojna

The visit begins with a train rather than a walk, and that sets the tone. Small open carriages run on a narrow-gauge track from the entrance deep into the system, passing through galleries lit as you go — it takes several minutes at a fair clip, it is cold in the moving air, and it is the part everyone remembers. Where the train stops, a guide leads a walking loop of a kilometre or so through the biggest chambers, over a bridge above a drop, and past formations that are genuinely difficult to take in at first: columns tens of metres high, curtains of flowstone, and a famous slender white pillar that has become the site's emblem. One chamber is used as a concert hall because the acoustics are exceptional and it holds a very large audience. The whole thing takes about an hour and a half and ends with another train ride out. Two practicalities matter more than anything else. It is around ten degrees inside all year regardless of the weather above, so take a jacket even in August — this catches out more visitors than any other thing about the site. And tours run at fixed times in multiple languages with limited capacity, so booking a slot in advance is close to essential in summer, which the trip-planning guide flags as one of the few fixed points worth locking in before you arrive.