Arapahoe Basin, CO. The Cirque Series is redefining what mountain running is all about. Starting from the base of this world-famous ski resort, you’ll rally up windswept ridges and glacial cirques to views of the Continental Divide then power down a quad-burning descent and break the tape into a festival of live music, food, drink, and vendor tents. The elevation is serious but the mileage is manageable, and the race is as hardcore as you want it to be.