Cloning world

The Sontarans used clone worlds to create soldiers. They did this by releasing gas into the air which gave the planet the ideal conditions to clone troops. This did however make that planet uninhabitable to other aliens. The Tenth Sontaran Battle Fleet tried to do this to Earth in 2009 but the Tenth Doctor stopped them. (TV: The Poison Sky)

In an alternate timelines, all Sontaran clone worlds had fallen to the Cybermen. (COMIC: Supremacy of the Cybermen