To achieve high thermal insulation without increasing weight, the Waterproof BODY X and BODY 2X undergarments are made of a hollow fiber polyester. Inspired by the structure of the fur of a polar bear who lives under extreme environmental conditions. It insulates far better than a regular fiber because it is hollow and filled with air. It is also designed to pull moisture away from the skin by capillary action and out to the exterior of the fabric where it easily evaporates. The combination of hollow fiber insulation and excellent wicking properties creates extremely warm and comfortable undergarments.