Caves of Han-sur-Lesse

The Caves of Han-sur-Lesse (in French: Grottes de Han-sur-Lesse or simply Grottes de Han) are a major Belgian tourist attraction (around 250-300,000 visitors per year) located on the outskirts of the village of Han-sur-Lesse.
The caves are the result of the underground erosion of a limestone hill by the river Lesse. For most of its length, it is a meandering river. Not far from Han-sur-Lesse, it abruptly plunges into a sinkhole known as the "gouffre de Belvaux" (English: Belvaux abyss), forcing its way under the hill over a distance of over as the crow flies before streaming through the cave complex known as the Grottes de Han.
Cave complex.
The caves have a constant temperature of and a high level of humidity. The largest room in the complex is called "La Salle du Dôme" (English: the Hall of the Dome); it is across with a vaulted ceiling that reaches .
Although modern exploration of the complex began in 1771, the cave has been known to locals for much longer. Bronze Age relics have been found inside, indicating use from at least the 5th century BC.
Access is only possible via a vintage streetcar, a remnant of the country's once extensive vicinal tramway system, which departs from the center of Han-sur-Lesse. The entry to the caves is about from the village. The guided tour takes about an hour to an hour and a half and includes a sound and light show in one of the largest chambers of the cave and an ending with a simulated cannon shot to demonstrate the cave's acoustic properties. Visitors used to exit the cave by way of a short boat ride at the point where the Lesse emerges on the surface, but since 2009 this has been replaced by a bridge.