Wikipedia teaches us: The caves are the result of the underground erosion of a limestone hill by the river Lesse. The river forces its way under the hill over a distance of over one kilometer as the crow flies. The caves have a constant temperature of 13°C (55°F) and a high level of humidity. 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 2 km 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 cannon shot to demonstrate the cave's acoustic properties.