The Radioactive Zone is a nightmarish dystopian cityscape that exists within an alternate reality created by Robotnik during the Robotnik Reigns Supreme arc. The city stands within the toxic wasteland once known as the Emerald Hill Zone, and is filled with industrial complexes that seem designed only to pollute for pollution's sake. What few residents the zone has live in constant fear of Robotnik's wrath, who sees and hears everything that goes on. Troopers are constantly on patrol and will execute citizens for little or no reason.
When Sonic the Hedgehog arrived in the Radioactive Zone, he found his allies Amy Rose and Johnny Lightfoot, who had no idea who he was due to Robotnik's tampering with history. Although Sonic rescued the two from a Trooper patrol, they abandoned him upon the arrival of Oscar the Pig's Death Squad. With no allies willing or able to help him, Sonic was forced to fight Robotnik's tyranny alone.
The Radioactive Zone never came into being, thankfully, since Sonic had goaded Robotnik into a fair fight and had Knuckles drain the villain's Chaos powers, which resulted in reality being restored to normal.