Grottes de Han

The Grottes de Han were cut by a river that gave up on the surface. The Lesse meanders quietly through Wallonia and then, not far from the village of Han-sur-Lesse, drops straight into a sinkhole known as the gouffre de Belvaux, the Belvaux abyss. From there it forces its way under the limestone hill for over a kilometre as the crow flies before running out through the cave complex.

What that erosion produced is one of Belgium's major attractions, drawing something like 250,000 to 300,000 visitors a year. The largest chamber, La Salle du Dôme, is 150 metres across with a vaulted ceiling reaching 127 metres. The cave holds a constant 13 degrees Celsius and very high humidity.

Modern exploration began in 1771, but local people knew it long before that, and how long is the interesting part: Bronze Age and Stone Age relics have been found inside, showing use from at least the fifth century BC.

Getting in is half the appeal and entirely unlike any other show cave. There is no walk to the entrance. Access is by a vintage streetcar, a survivor of Belgium's once enormous vicinal tramway network, which leaves from the centre of Han-sur-Lesse and carries you out to the cave mouth.

The visit ends the way the river does, and the tour is timed around the tram, so it pays to check departures rather than simply turning up. The site also runs a wildlife park on the hill above the caves.

The entrance is about 2 km from the village, which is why the tram exists. The guided tour runs an hour to an hour and a half and includes a sound and light show in one of the largest chambers, finishing with a simulated cannon shot fired to demonstrate the acoustics of the place. Visitors used to leave the cave by a short boat ride at the point where the Lesse comes back to the surface; since 2009 that has been replaced by a bridge.