A highly powerful explosive compound that is hard for transporters and sensors to pick out, used by assassins and saboteurs and by Sisko's crew against a Dominion ketracel-white depot.
Ultritium is one of the most destructive conventional explosives known in the Alpha Quadrant. In the Next Generation episode Manhunt, a pair of Antedean delegates arrive for a conference on Pacifica wearing robes lined with ultritium, intending to blow up the gathering, and are exposed only when Lwaxana Troi reads their minds.
In the Deep Space Nine episode A Time to Stand, Sisko's crew fly a captured Jem'Hadar fighter deep into Cardassian space and use a large charge of enriched ultritium to destroy the Dominion's main ketracel-white storage facility in the sector.
The compound is also mentioned elsewhere in Deep Space Nine and Voyager as a filling for warheads and demolition charges.