rate of two or three per week, were required to transport manufactured mine components to assembly depots in Scotland. Objectives The objective was to prevent U-boats from operating in the North Atlantic and preying on trans-Atlantic shipping. A similar barrage had already been placed across the English Channel, which had resulted in U-boats diverting north around Scotland. The North Sea Mine Barrage was intended to close this alternative route, and it also made it hard for the U-boats to get supplies. Mark 6 Mines The Mk 6 mine was a steel sphere containing a buoyancy chamber and of TNT. Each mine was constructed of two steel hemispheres welded together. A Toxyl bursting charge was cast into the lower hemisphere. Toxyl was a mixture of 60% trinitroxylene (TNX) with 40% TNT used because the United States Army controlled United States TNT production and would not release sufficient quantities for the naval mine barrage. For transport, the mine rested atop a box-shaped steel anchor approximately square. The anchor box had wheels allowing the mine assembly to be moved along a system of rails aboard the minelayer. The mine was connected to its anchor box by a wire rope mooring cable stored on a reel. The depth of the mine below the water surface was controlled by allowing the steel mooring cable to unwind from its reel as the mine was dropped from the minelayer until a sensor suspended beneath the anchor reached the bottom. The sensor locked the cable reel so the falling anchor would pull the buoyant mine below the surface; and the float extended the antenna above the mine. Each mine had two hydrostatic safety features intended to render the mine safe if it detached from its mooring cable and floated to the surface. The first was an open switch in the detonation circuit closed by hydrostatic pressure. The second was a spring pushing the detonator away from the explosive charge into the buoyancy chamber unless compressed by hydrostatic pressure. The mines were intended to be safe at depths less than . The mines contained a dry cell battery each with an electrical detonating circuit which could be initiated by any one of five parallel fuzes. Four of the fuzes were conventional horns in the buoyant upper hemisphere of the mine. Each horn contained a glass ampoule of electrolyte which would connect an open circuit if an ampoule was broken