NB recall that one way of detecting submarines is MAD (magnetic anomaly detector) e.g. local changes to background magnetic field caused by a large chunk of steel (a submarine).
Large physical deposits of ferromagnetic material would therefore affect the local magnetic fields: which in turn has implications for EMP...
G.W. They do use MAD (magnetic anomoly detection) in anti-submarine warfare (including dropping magnetic pellets on suspected subs to make them more detectable!)