Enligt noveliseringen var det en naturligt eroderad planet / yta som grävdes ytterligare.
The fleet of Star Destroyers stood off the white world. Spectacular and isolated, with a mean surface temperature varying from merely cold to permanently arctic, the planet had been altered: its mountains tunneled into, its glaciers hacked, and its valleys modified until it no longer resembled its original naturally eroded form. Those who had remade it had renamed it.
Starkiller Base.
Hollowed out of one snow-covered mountain was a central control facility. At its heart was a great assembly chamber that held hundreds of workstations and their attendant seats.