5mm, cold water thickness. Continuous innovation brings an unparalleled level of heat to the Furnace Ultra. The warmest wetsuit accessories in the world, this top shelf wetsuit bootie now features Billabong‘s exclusive Furnace Graphene. Innovative new graphene wrapped yarns combine with carbon fibers for a wetsuit that is lighter, stronger and warmer for longer. The pinnacle of wetsuit technology, Furnace Graphene is 50% lighter than traditional hollow fiber liners and 200x stronger than steel. Graphene lining and a thin yet durable sole design combine for a lightweight, ultra warm boot that offers a close board feel.