The Ultra-Trail du Mont-Blanc is a trail-running event that circles Mont-Blanc massif through France, Italy and Switzerland. Every year, at the end of August or at the beginning of September more than 10,000 runners join the race. The Ultra-Trail du Mont-Blanc is considered one of the largest and most difficult running race in the world. It is made of seven different foot races, four of which are ultra endurance trails. Some of these trails are several days long, in high altitude and with up to 25,000 meters of ascent.