"A naval mine is a self-contained explosive device placed in water to damage or destroy surface ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of, or contact with, an enemy vessel. Naval mines can be used offensively\u2014to hamper enemy shipping movements or lock vessels into a harbour; or defensively\u2014to protect friendly vessels and create \"safe\" zones."@en . . .