Mechanus

Mechanus was a jungle planet that served as the homeworld of the Mechonoids. It was also inhabited by Gubbage Cones, and had terrific metal deposits which the Mechonoids mined.

History
As Steven Taylor explained, humans sent Mechonoids to prepare Mechanus for colonisation, but Earth got caught up in interplanetary wars such as the Human-Draconian War, and the Second and Third Dalek Wars. No one was bothered with the abandoned Mechonoids, figuring they would run down or wear out, but the Mechonoids constructed their city and repaired themselves, continuing to function (PROSE: The Chase) even though the colonists never arrived. Steven Taylor crashlanded on the planet and was taken prisoner by the Mechonoids for several years, before being saved by the First Doctor and his companions as the Mechonoids and their city were destroyed by the Daleks, while the travellers escaped in a Dalek time machine and the Doctor's TARDIS. (TV: The Chase)

"The Government decided to open up this arm of the Galaxy, and sent out a fleet of ships to terraform any marginal planets. This place was one of them, and the shipful of colonizing robots came down to clear the landing sites, start building and generally make things as perfect as they could for the colonists. […] Earth got into the Draconian conflict, then the Third Dalek War. That ended the population expansions problem pretty drastically, and this arm of the Galaxy was promptly forgotten."

- Steven Taylor

Somewhat contrary to Steven's assessment, however, Jyl Stoker claimed in 2382 that the Mechonoids were outdated long before then, (PROSE: Fear of the Dark) although another source supported Steven's version of events. (PROSE: The Whoniverse)

Shortly after encountering the Mechonoids, the Daleks set the rogue planet Skardal on a collision course with their home world, but the Zerovians' robot agent 2K prevented Skardal from destroying Mechanus. (COMIC: The Rogue Planet, Impasse)

Behind the scenes

 * According to non-narrative material in DWBIT 3, an account was taken of the Daleks' battle with the Mechonoids during the former's mission to capture the Doctor.
 * In a Christmas ident when a Dalek is booking a flight from an Earth airport Mechanus is noted to be near Skaro.
 * Initial drafts had the planet named Mechon.