Thunderbolt was a British nuclear powered nerve gas missile.
Before the start of the World Peace Conference, UNIT was put in charge of its disposal at the bottom of the ocean. This was interrupted by the reappearance of the Master and a revolt at Stangmoor Prison, of which the Master took full charge. He planned to use escaped convicts to hijack the Thunderbolt and fire it at the Peace Conference thus starting World War III. The Third Doctor was able to activate its abortion mechanism, and UNIT destroyed the missile. (TV: The Mind of Evil)