South Island is the home of Sonic the Hedgehog and presumably Dr. Eggman. South Island is a lush green island located on Mobius that Eggman had originally planned on conquering until Sonic sent the both of them to the Mushroom Kingdom. Like its non-SMG4 counterpart, South Island consists of a few separate Zones.
Green Hill Zone
The most commonly seen part of South Island is the Green Hill Zone, which features checkerboard-patterned dirt, waterfalls, and vibrant green grass. This zone is well known for being the very first zone encountered in the very first Sonic Game.
Eggman's lair is presumably attached to or located near South Island. Outwardly, it resembles Green Hill Zone but with an Eggman emblem installed on the front. The interior of the lair is a purple brick palace resembling the Marble Zone and contains pools of magma. The marble tunnels lead to a throne/living room. The lair is heavily guarded by robots such as Omega, talking cannons, reprogrammed GUN robots, and Metal Sonic. Eggman's lair receives exactly one TV station that shows various animations/filming of Eggman dancing.