They built the city underground, beneath Robotropolis, where it remained hidden for all eleven years. When Griff brings Sonic, Sally, and Antoine to Lower Mobius, Sonic comments that they've heard of Mobians who built an underground city, but Robotnik seems unaware of its existence. All of the citizens of Lower Mobius appear to be either goat, pig, wolf or monkey species. They have attempted to continue on with a life as close to life in Mobotropolis was by having parks, growing flowers, opening shops and restaurants, etc. but none of the residents seem interested in fighting back.