Ilum

Ilum is an Outer Rim world covered with ice and coniferous forests, ill-suited for permanent habitation because of the cold. Its interior is riddled with caves whose walls are studded with kyber crystals, which were used in making lightsabers.

The Jedi Order considered the planet one of its sanctuaries: younglings were brought here so that each could find on their own, in the icy grottoes, a crystal that responded to their presence in the Force. The entrance to the caves opened only at a certain time, and the trial took place without the help of mentors.

After the fall of the Order, the Empire began industrial extraction of the crystals, destroying the ice masses and hauling away the raw material for its projects. Subsequently the First Order rebuilt the disfigured planet into Starkiller Base, turning its interior into a superweapon.