Mechanus

Mechanus is a barren, dusty world that was singled out by Humans as a potential colonisation world, but was never actually colonised. Probes sent by Humans to the planet containing Mechanoids to construct cities for them were intercepted on their way home by Daleks and destroyed, thus beginning the Second Galactic War. Meanwhile, the Mechanoids on the planet set to work building the cities which, in the absense of Humans, they lived in themselves, becoming a race in their own right.

The Mechanus Incident began here, which saw the annihilation of a thousand Reapers at the hands of the Mechanoids, as well as several incidents involving Daleks, which were also knocked back.