After admiring the bubbling Source du Lison, continue to Creux Billard, where the atmosphere is radically different. At more than 50m deep and bathed in a peculiar light, this corrie is an impressive spot. This cylindrical trough fault is a glimpse of sorts into the underground course of the River Lison, which resurfaces here. The site is well known to potholers, who sometimes venture into one of its caves: the Grotte Sarrazine.