You may be looking for New Megaopolis or Old Megaopolis.

Megapolis was one of the major Overlander cities on Mobius. It was home of two of the greatest scientific minds of Mobius's recent history - Julian Kintobor and Nate Morgan. The city had long been plagued with the problem of mass pollution by fossil fuels, something Nate Morgan had hoped to rectify. During his experiments to produce Power Rings as a clean, alternative fuel source, Julian sabotaged the project causing an explosion in the city, which resulted in Nate's exile. (StH: #65)

Years later during the Great War, Julian was sentenced to ten years in prison due to his plans to test live Overlanders on his weapon called the "Ultimate Annihilator". However, Julian escaped and fled to the Kingdom of Acorn where he would later use his scientific genius to provide technology and tactics which allowed the Mobians to win the Great War. Following the Overlanders' defeat, the majority of the citizens of Megapolis took off into outer space to find a new world to call their own, leaving the city uninhabited. Following his successful military takeover of Mobotropolis Julian, now calling himself Dr. Ivo Robotnik, sent his legions of SWATbots to take over the city and capture its citizens. The city had already been vacant due to the citizens taking off into space, preventing their capture. (StH: #72, #94)