An antimatter mine was a relatively small antimatter weapon used by Starfleet, the size of a small probe that contained a large quantity of antimatter. On board sensors detected close-range starships and the mine moved towards them. On impact, the containment fields within the mine collapsed, dispersing antimatter onto the ship's hull, causing a large number of instant annihilations, resulting in an explosion.
In 2369 the crew of the USS Enterprise-D used five hundred antimatter mines of varied sizes by order of Captain Edward Jellico to force a Cardassian fleet hidden into the McAllister C-5 Nebula to withdraw. The mines were placed surreptitiously by a shuttle and attached themselves on the hulls of the Cardassian ships. (TNG: "Chain of Command, Part II")