sentences:  In July 1939, before World War II had begun, the possibility of a Northern Barrage between the Orkney Islands and Norway (similar to the North Sea Mine Barrage of World War I) had been considered, however other alternatives were investigated after the occupation of Norway by the Germans in April 1940.Conventional mines of the era employed a contact-fuzed explosive charge within a buoyant shell suspended over an anchor attached by a wire rope.Use of such mines against surface ships required the mine to be suspended at a depth no greater than the ship's draft.