marine mine

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

• noun (Mil.) A military explosive device designed to be placed on or under the surface of a body of water, and to explode when ships pass nearby or come in contact with it. Its function is to destroy enemy ships or deny hostile naval forces access to certain areas of the sea, usually near the shoreline. Also called underwater mine and floating mine, and previously referred to as a torpedo (See torpedo{2} (a)).

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

• noun an explosive mine designed to destroy ships that bump into it