The Arctic Pro Extreme Boot from Muck is fleece lined with a stretch-fit Topline binding that fits snugly to the leg to keep warmth in and cold out. 8mm CR flex-foam bootie with four-way stretch nylon. 100% waterproof, lightweight and flexible. Bob-tracker molded outsole is rugged, aggressive and durable for maximum protection and stability. 2mm thermal foam underlay added to instep area for additional warmth. Reinforced toe with wrap-up bumper. Comfort range -60 degrees F to 40 degrees F.