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German U-boat operations in the North Sea needed to be dealt with.  To combat this issue, the U.S. Navy's Bureau of Ordnance developed the Mark 6 Antenna Mine.  To keep the project as a secret, the parts were made separately, then were built at different locations.   Originally developed in July 1917, the Mark 6 mine was formed by two hemispheres of steel welded together at the equator.   At St. Julien's Creek, Virginia, the goal was to manufacture 100,000 mines, each carrying 300 pounds of TNT.   The plant consisted of 22 buildings.  Construction began in late 1917, but due to bad weather, the plant did not open until March 1918.   The mines departed Norfolk, Virginia on a fleet of 24 cargo vessels.  The transportation consisted of two ships about every seven or eight days, which lasted 20-21 days for journey time.   The first vessels arrived at Scotland in May 1918. 

Image:   NH 123961:   Assembling Mines, Inverness, Scotland, September 1918.   U.S. Naval History and Heritage Command.