In September 1939, Germany began to deploy magnetic mines, «Hitler’s secret weapon number one,» for a naval blockade against Great Britain. The mines posed enormous dangers, given Britain’s heavy dependence on sea communications. As a defense measure, the English commenced the mass demagnetization (degaussing) of their ships by the end of November 1939. Degaussing not only saved England, but during the war years, Britain aided the Soviet Union in the formation of a demagnetization service by supplying the necessary measuring devices, non-contact magnetic trawls, and documentation on their use. After the war, special conferences on degaussing took place in the West, the proceedings of which were published in 1946. Russian articles and monographs on the demagnetization of ships began to appear in 1971, but claims about priority and the greater effectiveness of Soviet demagnetization often misrepresented historical realities. Our essay analyzes key moments in the history of naval degaussing in Great Britain and the Soviet Union in order to reveal the specific characteristics of Russian demagnetization and restore an accurate image of degaussing and its interesting and instructive history.