The poison sky

Earth's atmosphere is cleansed of Sontaran clone feed. (TV: The Poison Sky)

Clone feed was similar to amniotic fluid, but for Sontaran clones.

The Sontarans intended to use the ATMOS to fill the atmosphere of Earth with poisonous gas, which would become fuel for their new clones, and the planet would have become a clone world. However, the Tenth Doctor thwarted their plans by clearing the atmosphere. (TV: The Sontaran Stratagem/The Poison Sky)

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