SDVs carry a pilot, co-pilot, and four person combat swimmer team and their equipment to and from maritime mission objectives on land or at sea. The pilot and co-pilot are often a part of the fighting team. The SDV is lithium-ion battery powered and equipped with propulsion, navigation, communication, and life-support equipment. The batteries directly power the electric motor that drives the single screw propeller. Because they are all electric, SDVs are extremely difficult to detect using passive sonar, and their small size makes them hard to detect using other means. The Mark 8 Mod 1 SDV can deliver four fully equipped SEALs to the mission area, be "parked" or loiter in the area, retrieve the SEALs, and then return to the launch site. The SEALs sit upright in the Mark 8, with the pilot and co-pilot/navigator facing forward and the other four facing aft. For air, the SEALs rely on their own air tanks or rebreathers, supplemented by compressed air tanks on the SDV. The crew and passenger compartment in the Mark 8 is small, cramped, and pitch black except for the dim lights of the instrument panel; SEALs describe riding in an SDV as like "being locked in a little black coffin deep under the water." As such, many SEALs are hesitant to enter an SDV, and only 10 percent are qualified to travel in one.