A cloaked spatial mine laid around a world claimed by the Romulan Star Empire, one of which clamped on to the hull of Enterprise NX-01 in Minefield.
The mines appear in the Enterprise episode Minefield, the story of Earth's first contact with the Romulans. Enterprise approaches an uncharted planet and is torn open by an explosion from an invisible mine. A second mine then attaches itself magnetically to the hull without going off.
Malcolm Reed goes outside to disarm it, and one of its anchoring spikes shoots through his leg and pins him to the ship. Archer joins him and works through the arming mechanism under his guidance while two Romulan warships decloak and order Enterprise to leave. The humans hear the Romulans' voices but never see them.
When the mine rearms, Archer has the section of hull plating it is fixed to cut loose, frees Reed, and the two shelter behind a shuttle hatch as it detonates.