The MK-21 diving helmet is currently the U.S. Navy's primary diving helmet for salvage and deep water diving. It is also utilized for underwater ship's husbandry on surface ships and submarines. The design was taken from the popular Superlite 17. The helmet weighs approximately 27 pounds dry and is neutrally buoyant in the water. The shell is made of fiberglass with brass weights. The regulator is similar to a SCUBA second stage and is fitted to an oral nasal mask on the inside of the helmet to prevent CO2 buildup. The helmet is supplied with air through the diving umbilical which also allows for two way communication with the supervisor on the surface.