South Island
From Sonic Retro
South Island is an island somewhere in the Pacific Ocean. It is not rooted in the ground, and thus floats around; because of this, it does not appear on most maps.
The original Sonic the Hedgehog takes place on South Island, as well as Sonic Chaos. It hasn't been returned to since (officially).
The Chaos Emeralds ended up there for many years before they were discovered by Sonic and Dr. Eggman.