This giant limestone cave hollowed out by the Lesse (over 15 km) served as a refuge from the late Neolithic period to the 18C. It has huge concretions (5m-long Minaret Stalagmite) and magnificent galleries: the Salle des Mystérieuses (crystal palace); the Salle d'Armes, which the Lesse runs through and he setting for a sound and light show; the Salle du Dôme, where you see a torch-bearer shoot down the enormous chamber. Boats take visitors back to daylight at the Trou de Han.