As if swimming 1.2 miles, biking 56, and running 13.1 more wasn’t difficult enough, try doing it at 10,291 feet. Yet the 106° West Triathlon, named for the race’s longitudinal coordinate, isn’t just making waves as the loftiest outdoor triathlon. When it takes place on September 10, the grueling contest will also mark the first time Dillon Reservoir has been open to swimmers. (Click here for less taxing waterfront activities.) Dillon’s recreation committee has kept the lake off-limits to backstrokers, but not watercraft, for 52 years for safety reasons: The reservoir only warms up to 60 degrees, and hypothermia can start to set in if you spend more than two hours in water temps below 70. After a nine-year approval process, however, triathletes came away with a victory. Medical professionals will be on-site during the race in case competitors succumb to the chilly surf. So make sure to bring a wet suit—because the custom belt buckle finishers receive is a much better souvenir than a hospital bill.