In 2152, Enterprise NX-01 encountered a field of these mines, which were cloaked, surrounding a planet that the Romulan Star Empire had annexed. The explosion of one such device caused a significant amount of damage to Enterprise's port forequarter. Another mine lodged itself on the hull, dangerously close to the ship's starboard impulse reactor, but failed to detonate. LieutenantMalcolm Reed and CaptainJonathan Archer attempted to disarm the mine, but were unable to do so before two Romulan Birds-of-Prey arrived, demanding that Enterprise leave the system. Archer and Reed managed to separate the section of hull plating where the mine was attached to the rest of the ship, and detonate the mine at a safe distance from the vessel. (ENT: "Minefield")
At a yield of 250 tons of TNT, the Romulan tricobalt mine was approximately twice as powerful as 23rd centurycobalt bombs mentioned in TOS: "Obsession", said to be 1/10,000th of the yield of an ounce of antimatter or approximately 122 tons of TNT. 
In the final draft script of "Minefield", the Romulan mine was described as "a large, metallic spheroid" and the script went on to say, "We'll learn that this is an alien orbital mine [....] The mine's surface is facetted with octagonal panels. A number of javelin-like spikes protrude from the mine at odd angles." The script referred to the process of the spikes attaching to the hull of Enterprise as being done "magnetically." A subsequent script note stated that, while the quantum beacons are active, "Cloaked mines are not visible unless on the viewscreen." The script also referred to the yield of an exploding mine as "about fifty kilotons," rather than a quarter of a kiloton.
As no parts of the Romulan mine were added with CGI in post-production and everything it did had to instead be shown with practical machinery, creating the mine turned out to be a challenging project. "We had two weeks to come up with this prop," recalled Set Decorator James Mees. "After much searching, I found a large, used fiberglass buoy, the kind used to mark channels for ships. We ended up cutting it open to add the Romulan 'electronics' and the spike that you see in 'Minefield,' along with the machinery to make it work." (Star Trek: Communicator issue 152, p. 35) In ENT Season 3, the prop was reused for the core of the Xindi weapon. ("Countdown" text commentary, ENT Season 3 DVD special features)