Here, we see that the great American military inventor James B. Glennon actually invented a magnetic mine as early as 1922(he filed for the patent in 1925, which was allowed at the time):
https://patents.google.com/patent/US1531550A
During the War, he would work with Robert Park, one of the greatest engineers to ever live, to dramatically improve the invention and devise defensive strategies to counter them.
https://patents.google.com/patent/US2400549A/en