The Pyrenees mountain range forms one of the best defined natural borders in the world. It stretches from the Mediterranean Sea in the east to the Atlantic Ocean in the west. Until recently, this natural barrier between Spain and France was almost impassable for several months of the year except for a few roads near the Mediterranean and a road on the Atlantic coast. Some passes can regularly blocked by snow in winter despite the presence of high-altitude tunnels. Perhaps the most famous of these passes is the Col du Tourmalet which sits at an altitude of 6938 feet (2115 meters) and is a regular part of the Tour de France circuit.