Sonic, Knuckles, Rotor, and Cobar outside the Tachyon Chamber
The Tachyon Chamber was a device built by Dr. Eggman in an alternate universe. So powerful was the device it could theoretically transport someone back in time. The device was hidden beneath the desolate and toxic Badlands in an underground bunker supposedly capable of sheltering the Tachyon Chamber from any threat. The Tachyon Chamber was studied by Rotor and Cobar, and used by King Sonic to go back in time and undo the damages he caused to the space-time continuum. Those located within close proximity to the chamber when it was used retained their memories even after the timeline was altered. (StH: #144, #166)