Sonic Mars was a concept game in development for the Sega 32X, which was later reworked into Sonic X-treme. The 32X version was cancelled, probably due to a combination of the add-on's limitations and its ultimate failure in the market.
Unlike most Sonic games at the time, this game was going to be based on the Sonic SatAM cartoon. It was to include three playable SatAM characters: Sally, Bunnie Rabbot, and Antoine, as well as Tiara Boobowski (Tiara Cyberooski in this game, who would later be moved to X-treme). This was also going to be the first 3D polygonal Sonic game. The SatAM theme was dropped eventually, due to disinterest from the development team.
In 2007, an early script of Sonic Mars by Michael Kosaka was leaked. Interestingly enough, on the list of upcoming games are Sonic the Hedgehog 3+ and Sonic Stadium. Another script  was also leaked by Don Goddard. The cover page of the Michael Kosaka script is dated May 19, 1994 and the Don Goddard script was created later.
"While Sonic is away checking on a security alarm in a remote part of the Great Forest, his pals (Sally, Bunnie, Tails and Knuckles) have discovered a strange message from within one of Robotnik’s super computers. [Dr. Robotnik is trying to take over a computer VR world (Micro Mobius) and the message is a plea for help from its peaceful inhabitants.] Sonic returns to Knothole to discover that Robotnik has captured his friends and taken them into his VR world. Sonic attempts to save his friends and thwart Robotnik’s plan to “reformat” Micro Mobius."
- Sonic Mars plot summary