Dicobalt was a high-yield explosive.
In 2153, pirates attached a beacon with a dicobalt explosive to the outer hull of the ECS Horizon. A similar device had been attached to the ECS Constellation a few weeks earlier. (ENT: "Horizon")
Coridan insurgents on the planet Coridan were known for taking hostages to acquire weapons, including dicobalt explosives. (ENT: "Shadows of P'Jem")
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