South Island is a region of Mobius that contains many of the Hill Zones - Green Hill, Emerald Hill etc. It is the home of Sonic the Hedgehog and the Freedom Fighters. The north of the island contains the Frozen Zone; the Frozen Zone is also the location of the North Cave, Sonic's hiding-place for the six Chaos Emeralds.
The video games Sonic the Hedgehog, Sonic Chaos and Sonic the Hedgehog 2 (8-bit) are said to take place on South Island. If this is the same as the Sonic the Comic continuity, it would suggest that Marble Zone, Spring Yard Zone, Labyrinth Zone, Starlight Zone, Scrap Brain Zone and Turquoise Hill Zone are all on South Island. However, since Emerald Hill is said to reside here, it can be assumed that the geography of the two canons do not match.
South Island is also the larger of the two islands that make up real-world New Zealand.
For appearances of individual Zones, see their respective pages.