Naval mine
Clearing WWII aerial mines

Between 600,000 and 1,000,000 naval mines of all types were laid in WWII. Advancing military forces worked to clear mines from newly-taken areas, but extensive minefields remained in place after the war. Air-dropped mines had an additional problem for mine sweeping operations: they were not meticulously charted. In Japan, much of the B-29 mine-laying work had been performed at high altitude, with the drifting on the wind of mines carried by parachute adding a randomizing factor to their placement. Generalized danger areas were identified, with only the quantity of mines given in detail. Mines used in Operation Starvation were supposed to be self-sterilizing, but the circuit did not always work. Clearing the mines from Japanese waters took so many years that the task was eventually given to the Japan Maritime Self-Defense Force.

For the purpose of clearing all types of naval mines, the Royal Navy employed German crews and minesweepers from June 1945 to January 1948, organised in the German Mine Sweeping Administration (GMSA), which consisted of 27,000 members of the former Kriegsmarine and 300 vessels. Mine clearing was not always successful: a number of ships were damaged or sunk by mines after the war. Two such examples were the liberty ships Pierre Gibault which was scrapped after hitting a mine in a previously cleared area off the Greek island of Kythira in June 1945, and Nathaniel Bacon which hit a minefield off Civitavecchia, Italy in December 1945, caught fire, was beached, and broke in two.
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