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, such as the crystal Salle des Mystérieuses, the Salle d'Armes, crossed by the Lesse and featuring a Son et Lumière show, the Salle du Dôme and Salle des Draperies, with a vault spiked by marbled stalagtites. Boats take visitors back to the Trou de Han.