- For the version of this location after the Super Genesis Wave, see Onyx Island (Archie).
The island stands in the sea at a slanted angle with part of its surface submerged as a result. It is home to the ruins of a city, including a library. The island is home to interesting artwork, including a painting of the Future Freedom Fighters and a massive statue resembling Lara-Su, one of that group's members. From Onyx Island, it is possible to see the burning remains of the Space Colony ARK in the distance.
At some point, the island fell from the sky and into the ocean. Some two hundred years after the time of Sonic the Hedgehog, the island became inhabited by Knuckles the Echidna's descendant Edmund, who described himself as a "failed Guardian." While Silver the Hedgehog does not live on the island, it seems he is a frequent visitor, often visiting the island's library that in his research to identify a suspected past traitor among the Knothole Freedom Fighters.