This hack, by Carretero, modifies the original Sonic the Hedgehog for the Sega Genesis, offering a unique gameplay experience. While specifics of the changes are not fully listed, it has been described to include backward play and a return to origins, which likely refers to modifications to level design, possibly restoring elements or challenging the player with altered game mechanics. Carretero's modifications likely affect the original game's level structure and flow.  As a ROM hack, it's a fan-made project, showcasing the dedicated community surrounding classic games.

Sonic the Hedgehog (USA, Europe) [Hack by Carretero v2.0] (~Sonic - Return to the Origin) (Backward Play)

Sonic the Hedgehog (Prototype)