The Tunnel rooms.

If your not careful and clip out if reality in wrong areas, you'll end up in the Tunnel Rooms, where it's nothing but the stink of Coal, the madness of Mid-Grey, and A endless background noise of fluorescent lights at maximum hum & buzz, and approximately 600,000,000 square miles of randomly segmented empty rooms to be trapped in. And Help me god, If you make any sort of noise. It heard you.. Loud, and clear.