Tom gives the impression (accidentally, I hope) that magnetic mines are pulled toward the ship by the magnetic fields. No. The detonation mechanism of the mine is triggered when a large, magnetically active thing (such as a big steel ship) gets close enough to the mine (which is usually anchored to the sea bed), and the mine goes Bang. Unlike a contact mine, which requires physical contact (like those WW2 mines with all the horns sticking out) or an acoustic mine, which is triggered by noise, usually propeller noise.