The Cave of Niaux in the Ariège department is one of the few still accessible to visit in France. It is part of an 8.7 miles (13 km) underground network carved into a limestone massif, only a section of which is open to the public. The remaining part ending on the other side of the mountain cannot be visited and can only be accessed by diving. The underground network made it possible to discover more than 500 human footprints as well as five animal illustrations. The walls of the Cave of Niaux are decorated with three bisons, a horse, and a weasel, the representation of which constitutes the only one ever observed in prehistoric cave art.