Diobath was an artificial planet composed of various environments created by a company called Sierra Logistica to conduct their experiments. Over time, the rodent-like inhabitants (part of the original experiment) evolved into peaceful creatures with the company's initials marked on their foreheads. They sheltered in the underground systems. Over many centuries, after the company ceased trading, the planet's systems began to break down and decay. When the bio-dome began to collapse and chunks of the planet's "sky" fell to the ground, off-worlders like the Gorolox arrived to plunder the planet.
The Eleventh Doctor repaired the underground systems (used as a safehouse by inhabitants like Mersal and Kilo). The invading Goroloxes left, fearing that the planet was being protected by their sworn enemies. (COMIC: The Sky is Falling!)
