magnetic mine

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A marine mine detonated by a mechanism that responds to a mass of magnetic material, such as the steel hull of a ship.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun military A mine which responds to the magnetic field of a target.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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