Osselle Cave is concealed behind the cliff overlooking a meander in the River Doubs. Discovered in the 13C, it has been open to visitors since 1504. Its dry galleries served as a refuge and chapel for priests during the French Revolution; a clay altar can still be seen. The skeleton of a cave bear has been reconstructed from bones found under the scree. Over 8km, following the ridge of the mountain, 1 300m of long, regular walkways have been laid out in the site.