Fusion bomb

Fusion bombs were a devastating weapon. They were tested in the 23rd century. If detonated in sequence, their effectiveness increased logarithmically. The effects were: They were banned before any larger combination could be tried.
 * One bomb - leveled a city and could kill upwards of 50 million people
 * Five bombs - total destruction of a planet
 * Eight bombs - solar system-wide, shattering every solid planet and igniting the gas giants

Adjudicator Medford, believing the Time Lords were a threat to Earth, filled the Machine with more than twenty (which could potentially annihilate a galaxy) and had the Fifth Doctor and Patience send it back to ancient Gallifrey. It did not reach its destination, thanks to the Seventh Doctor. (PROSE: Cold Fusion)