Sonic X-treme was a 3D Sonic the Hedgehog game for the Sega Saturn. The concept was truly unique; to further the traditional Sonic "go-anywhere-or-run-through" formula, every level would be designed in a rather tube-like fashion; Sonic would be able to walk onto walls, thus changing the direction of gravity and the rotation of the level itself. In addition, an unusual, fish-eye lens-styled camera was put into place so players could see more of their surroundings at any one given time.
Unfortunately, it was canceled because of many, many internal problems. The game was being developed by Sega Technical Institute, a US-based developer that had worked on such games as Sonic The Hedgehog 2, Sonic Spinball and Comix Zone. Originally it was based on the engine Sonic Team's at-the-time most current work - NiGHTS into Dreams - but Yuji Naka discovered this and had their usage of the engine pulled. As of such two separate engines started work; one was for the main levels, while the other was for the bosses.
Later on, a Sega of Japan representative came over to check on the game's progress, and was so impressed by the boss engine he requested the entire game be made on that. By now the team was running short on men, and it all had fallen on one man, Chris Senn, to finish it up before the Christmas deadline so as to go up against Super Mario 64 alongside NiGHTS. However, despite his putting 200% into the project, he literally worked himself sick and had to stop before it became any worse. Sadly, that was the nail in the coffin as Sega of America pulled the plug then and switched to plan B: a Saturn port of Sonic 3D Blast. Instead of releasing that and then continuing work on X-treme, that was it; it was dead.