floating mine

English Thesaurus

1. an explosive mine designed to destroy ships that bump into it (noun.artifact)

hypernym	:	mine,
definition	:	explosive device that explodes on contact; designed to destroy vehicles or ships or to kill or maim personnel (noun.artifact)

hyponym	:	magnetic mine,
definition	:	(nautical) a marine mine that is detonated by a mechanism that responds to magnetic material (as the steel hull of a ship) (noun.artifact)