Phantom Zone
In "Action Comics" #874, the Phantom Zone vanished. "Action Comics" #886 offers a possible explanation as to the Phantom Zone's disappearance, the theory being that the Phantom Zone was actually the mythical Nightwing, counterpart to the Flamebird, imprisoned in an altered state of being. Having chosen a new Avatar, Chris Kent, who was freed from the Zone, he too would have been freed from his shackles, thus causing the Phantom Zone to cease to exist.