Gaster's greatest achievement. The Core has been falling apart for years now, suffering not just from lack of maintenance and the foul power of the Glitch, but heavy damage from Mettaton's rages. Large parts of its machinations have fallen from its body and into the lava fields that power it, and many of its rooms are inaccessible, blocked by fallen pipes and destroyed walls. The ice flows steadily still to cool the reactor, provided by the yet operational ice machine in Snowdin Village, but this will not save the geothermal energy producer forever.
Mettaton stalks the sparking, shaking hallways obsessively, searching always for prey and chasing the phantom laughter of the one other remaining inhabitant. He never leaves, no longer capable of remembering anything beyond the machine.