 Soon after the outbreak of war in September 1939 , the Germans began a naval mine campaign against Britain . The results were devastating . Nineteen ships totaling 59 @,@ 027 tons were sunk by mines in October , and 27 totaling 120 @,@ 958 tons in November , along with the destroyer HMS Blanche . Many more ships were damaged , including the cruiser HMS Belfast . The nature of the mines was initially unknown , but on 23 November 1939 , a bomb disposal team under Lieutenant Commander J. G. D. Ouvry recovered an intact aerial mine from a mudflat at Shoeburyness , and the threat was revealed to be a magnetic mine . 
