The Muck Arctic Sport Boot provides comfort and warmth in even the most extreme conditions. It has all the standard Muck Boot features plus: Stretch-fit topline binding snugs leg to keep warmth in and cold out. 5mm CR flex-foam bootie with four-way stretch nylon,100% waterproof, lightweight and flexible MS-1 molded outsole is rugged, aggressive and durable for maximum protection and stability. Double reinforcements on the instep, heel and Achilles areas. 2mm thermal foam underlay added to the instep area for additional warmth. EVA molded midsole with contoured footbed. Reinforced toe. Added toe protection with a wrap-up bumper. MPN: ASP-404