A highly reliable style of footwear that prevents frostbite in alpine environments. Early mountaineering boots were crafted from hobnailed leather, which freezes. The solution was to use a plastic shell fitted with an easily removable synthetic liner that can be taken into your sleeping bag at night and kept warm. The first double boot to reach the U.S. was the Koflach Ultra, in the 1980s. These days, double boots are frequently paired with lithium-powered heaters to keep a climber’s feet toasty in temperatures down to 60 below.