Mechanus is the jungle homeworld of the Mechanoids.
Shortly after encountering the Mechanoids, the Daleks set the rogue planet Skardal on a collision course with their home world. The Zerovians' robot agent 2K prevented Skardal from destroying Mechanus. (DC: The Rogue Planet, Impasse)
Humans sent Mechanoids to Mechanus to prepare the planet for colonization, but Earth got caught up in an interplanetary war and the colonists never arrived. Steven Taylor crashlanded on the planet and was taken prisoner by the Mechanoids. The Mechanoids and their city were destroyed by the Daleks, and Steven departed in the Doctor's TARDIS. (DW: The Chase)