Mobius, or Planet Mobius, is a planet from an alternate dimension to the Mushroom Kingdom, and the main setting of the Sonic the Hedgehog franchise, apart from Earth. It is the homeworld of Sonic the Hedgehog and Dr. Eggman, in addition to Tails the Fox, Knuckles the Echidna, Amy Rose, Shadow the Hedgehog, Rouge the Bat and several others. It appears in the Sonic the Derphog series, but the most seen of it is it's capital island South Island and it's primary level Green Hill Zone, where Sonic and his friends hang out.
In Sonic the Derphog: Eggventure (GET IT!?), Sonic spends the day being harassed by Eggman until Sonic accidentally activates an inter-dimensional teleporter that transports them into the Mushroom City, leaving Mobius behind. However, they have made their way back to it by Sonic the Derphog: The Item Brick, having managed a way to travel back and forth between the Mushroom Kingdom and Mobius as of Sonic the Derphog: Egg and Peach.