Reactive Factory is the fourth track in Sonic R. It is a scientific lab owned by Dr. Eggman. All of the unlockables except for Eggman and Super Sonic were created here. Metal Knuckles is unlocked here and it houses two Chaos Emeralds.
The track is a puzzle-laboratory. There are many confusing turns and twists, and is considered "hard" to many players. There is a shortcut, though, which helps inexperienced players. A large machine is housed in this level that gives free rings, allowing players to obtain Chaos Emeralds and Sonic Tokens easier.
The instruction manual states the following:
"Here you are transported into the future! Enter the factory of tomorrow, a completely automated wonder. There is lots of room to run, but there are also many hidden surprises and traps. What appears to be at first a simple course is really the hardest of the four courses."
The main theme song in this course is Work It Out.