Wiktionary
n. (context military English) An explosive device placed in the water and intended to destroy or deter vessels. (See mine.)
Wikipedia
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—to hamper enemy shipping movements or lock vessels into a harbour; or defensively—to protect friendly vessels and create "safe" zones.
Usage examples of "naval mine".
The Argentines were developing a new and top secret naval mine, made of plastic, rendering it difficult to detect using conventional minesweeping techniques.
Dozens of corridors emptied into the area, like detonator horns on an old-fashioned naval mine.