Sonic X-treme

Sonic X-treme was a platform game developed by Sega Technical Institute from 1994 until its cancellation in 1997. X-treme would have been the first fully 3D Sonic the Hedgehog game and the first original Sonic game for the Sega Saturn. Its concepts built on past Sonic games while introducing elements to take Sonic into the 3D era of video games.

In 2006, a copy of an early test engine was sold at auction for US$2500 to an anonymous collector. After a fundraising project by the ASSEMBlergames website community purchased the disc from the collector, the disk image was leaked on July 17, 2007. In February 2015, fans obtained the Sonic X-treme source code and created several playable builds.