The GR 400 circumnavigates the Puy Mary Volcano by loops, the itinerary presented here is the biggest loop one can make on this trail. You mainly follow the ridgeline for a while, then go down in the valley to go back up to the next ridge line. It´s a beautiful itinerary with a lot of open views, a lot of springs where you can find some clear water, and trails that are just a joy to run on.
