During World War I, Germany laid more than 43,000 mines that claimed some 500 merchant vessels. The British Navy lost 44 warships and 225 auxiliaries to mines. The purpose was to interrupt the flow of supplies to Britain and to hamper the British fleet. Mines were most often set near harbors and inlets, as these were areas of high traffic. Great Britain reacted to the threat by setting up an intelligence network to identify, map, and distribute information about the location of mines.
One such map produced by British Naval Intelligence is shown here from the William Rea Furlong map collection, which is held by the Geography and Map Division. The once secret document depicts the general situation of minefields surrounding Great Britain on August 19, 1918. It was not meant for navigating through a specific minefield, as those were mapped separately at greater scale that showed more detail. The map also illustrates areas where the British had placed mines to restrict the movement of German military and commercial vessels.
As the legend indicates, areas hatched in red are dangerous due to moored British mines. Numbers in red within the black rectangles indicate the number of moored enemy mines swept up and destroyed in that area in the two-week period between August 5th and August 19th, 1918. The red asterisk-like symbols indicate harbors that have a swept channel through which ships can enter and leave safely. Closer looks at the map show the extent of naval minesweeping operations, from the English Channel to the north coast of Scotland:
Secret maps, such as this one, were essential to the British Navy to confidentially report on the status of German minefields to commanders and policymakers. This map in particular boasts of the British Navy’s successful minesweeping operations that minimized the threat posed by German mines.