The Collaborative International Dictionary of English v.0.48:

magnetic mine \magnetic mine\ n. (Mil.)
   A marine mine that is detonated by a mechanism that detects
   the nearness of magnetic material (as the steel hull of a
   ship). A magnetic counter mine may allow the passagw of
   several ships before detonating.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

magnetic mine
    n 1: (nautical) a marine mine that is detonated by a mechanism
         that responds to magnetic material (as the steel hull of a
         ship)