naval mine

explosive planted in the sea and intended to damage ships or submarines

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