and included various "Marks" such as the Mark V, VI, and VII. Intermediate numbers were assigned to some vehicles that never made it off the shop floor. All were of flooded design. The first SDV to be operationally deployed was the Mark VII, which entered service in June 1972 after being tested between 1967 and 1972. It could carry three SEALs plus a pilot sitting in compartments fore and aft. It had a hull made from fiberglass and non-ferrous metals to hinder detection and was powered by a silver-zinc battery attached to an electric motor. The Mark VIII SDV, the model that is still in use today, began to supplant the Mark VII starting in 1983. The wet vehicle SDV program (officially named the Swimmer Delivery Vehicle, later re-designated the SEAL Delivery Vehicle after the Swimmer Delivery Vehicle Teams were renamed SEAL Delivery Vehicle Teams) currently centers on the Mark VIII Mod 1. The SDV was first developed in 1975 for use among UDT/SEAL teams. The early Mark 8 Mod 0 SDVs had a PRC104 UHF radio for use underwater. The newer model Mark 8 Mod 1 has a dual sliding canopy and quick release hatch. Design 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 silver-zinc 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