Old Megaopolis is the remains of an ancient city on Mobius similar to New York City. The Statue of Liberty's remains are located near there, suggesting it is located in the same location as New York was prior to the Xorda's attack.
Old Megaopolis was used by Dr. Eggman as a hideout when he constructed a new robotic body for himself after his old one was destroyed. Almost a year later Old Megaopolis' harbour was used by Eggman, where he planned to launch two nuclear missiles at Knothole and Station Square from his battleship. Before the missiles could launch however his battleship was taken out by Bunnie Rabbot. (StH: #116, #130, #133)