Robotnik test fired the Death Egg in the past causing the present to be a wasteland.
Dust Hill: a placeholder name for Madness Mountain, which was reskinned into Mystic Cave.
Rock World: a nuclear winter where Sand Shower would've been.
Oil Ocean: an oil plaform off the coast of South Island.
Death Egg: Robotnik's space station launches from the mountains of South Island. It contains a time machine.