Sonic Robo Blast 2 is a free 3D Sonic the Hedgehog fan-game built on a heavily modified fork of the Doom Legacy source port of Doom. It is closely inspired by the original Sonic games from the Sega Genesis, and attempts to recreate their classic design in 3D with robust multiplayer and mod support.
Running about on a trans-gravity conveyor belt - in this section, you immediately switch gravity upon jumping, forcing you to weave your way through the room between the conveyor belts, lest you fall to your death in either direction.