The Thermolite polyester at the heart of DeFeet's Thermeator Hollow Core 6 inch Socks are a synthetic version of merino wool. If extraterrestrial sheep aren't just fantasies of our winter addled minds, they'll probably be covered in something like it. Thermolite works by creating pockets of trapped air in its hollow core yarn, which serve as extra insulation between your feet and the very real cold of winter, keeping your feet from freezing into numb clubs during those cold weather base miles. Thermolite is also breathable, it wicks, and it dries quickly, and the mesh instep only enhances these qualities. The gently compressive arches add support, and a touch of Lycra gives the socks an accommodating stretch.