The train, a small, open-sided electric locomotive, the carriages just wide enough for two adults sitting knee-to-knee, lurches forward and plunges into the darkness of Postojna Cave, the headlamp illuminating a tunnel of limestone that twists and narrows and opens suddenly into chambers the size of cathedrals. The railway, the first underground railway in the world built for tourism, opened 1872, electrified 1924, still running on much of the original route, is not a gimmick. It is the only practical way to reach the heart of a cave system that extends 24 kilometres, the longest show cave in Europe, and the 3.7 km train ride through the darkness, the stalactites flashing past inches from your head, the roar of the Pivka River echoing from somewhere deep in the rock, is as close to time travel as tourism gets.

Postojna Cave: 140 Years of Underground Discovery

The Railway (opened 1872): Before the railway, visitors to Postojna Cave, the cave has been a tourist attraction since 1819, when the local lamplighter Luka Čeč discovered the inner chambers, walked the full 3.7 km on foot, carrying torches. The railway, initially pushed by the cave guides (who, by all accounts, were tired of pushing tourists), was a sensation: the first underground railway in the world, a triumph of engineering in a space where the ceiling is, in places, less than a metre above the carriage roof. The original locomotives were steam-powered, the smoke, the noise, the complete unsuitability for an enclosed space, and were replaced by electric locomotives in 1924. The current trains, silent, electric, the carriages open to the cave air (a constant 8-10°C year-round, bring a jacket), carry 800,000 visitors annually. The journey: 2 km by train into the heart of the mountain, then 1.5 km on foot through the most dramatic chambers, past the Brilliant (a pure white stalagmite, 5 metres tall, the symbol of the cave), the Concert Hall (the largest chamber, 40 metres high, 3,000 square metres, the acoustics so good that concerts have been held here, the capacity 10,000 standing, the temperature and humidity controlled, the experience of hearing an orchestra in a cave unique), and the Russian Bridge (built 1916 by Russian prisoners of war, the single-span bridge crossing a 20-metre-deep chasm, the work conducted in silence at 8°C, the engineering a lasting testament to forced labour). The human fish (Proteus anguinus, the olm, the blind, pale, cave-dwelling salamander that can live for 100 years and survive without food for a decade, the largest cave-dwelling animal in the world, found only in the Dinaric karst of Slovenia, Croatia, and Bosnia) is the cave’s most famous resident. The olm tank in the Concert Hall is the most-photographed exhibit. The olm does not seem to care.

Predjama Castle (9 km from the cave): The castle, built into the mouth of a cave, the rock face 123 metres high, the castle clinging to the cliff like a stone limpet, is the companion attraction. The castle has been occupied since the 12th century, but its most famous resident was Erasmus of Lueg (Erazem Predjamski), a 15th-century robber baron who defied the Holy Roman Emperor Frederick III for a year, using the cave system behind the castle (a network of tunnels and passages, the natural karst the ultimate siege-proof larder) to outlast the besieging army. Erasmus was eventually killed, not by starvation or assault, but by a cannonball fired through the toilet window, the weakest point in the castle walls, a piece of intelligence reportedly supplied by a treacherous servant. The toilet is still there. The cannonball is not. The story is better than the castle, and the castle is excellent.