The Brown Hill Zone is an alternate version of the Green Hill Zone from the novel Sonic the Hedgehog in the Fourth Dimension. It was created after Dr. Robotnik used the Mark-0 Gauss-Coyne prototype time machine to change history.
The Brown Hill Zone is similar in many ways to the Green Hill Zone, but influenced by years of Robotnik's pollution. The known differences are as follows:
- As the name suggests, the verdant green hills of the Green Hill Zone are, in the Brown Hill Zone, brown.
- Whereas the sand in the Green Hill Zone beaches is fine and white, like sugar, in the Brown Hill Zone the sand is coarse, rough and dirty grey in colour, and contains sharp stones (at least some of which have "Copyright © Robotnik Industries" etched on them).
- The water of the Brown Hill Zone is murkier and more unpleasant than that of the Green Hill Zone.
- The air of the Brown Hill Zone isn't clean, but smells like the Oil Ocean Zone.
- The palm trees are less tall and less healthy than those of the Green Hill Zone.
- In the Green Hill Zone the weather was generally pleasant and it never hailed; in the Brown Hill Zone it rains and hails all the time.
- The lake of the Brown Hill Zone contains something "huge, black and scaly".
Sonic's friends still live in the Brown Hill Zone, but the group has changed slightly: Sally Acorn, Johnny Lightfoot, Joe Sushi, Flicky, Chirps, and Tux are still present, but Porker Lewis has been replaced by Carrie the Coypu, who is also Tails' fiancée. Also, Eric the Echidna, who resembles Sonic except for his long nose and tongue, claims to be a local hero although he actually works for Robotnik. Both Carrie and Eric have "©" etched on their feet.
After finding an old, rusty version of the Mark-0 Gauss-Coyne prototype, Sonic and Tails witness the timeline shifting and the Green Hill Zone being replaced with the Brown Hill Zone. None of their other friends realise the differences.
While Tails is (literally) held behind by Carrie, Sonic runs off to investigate and discovers a cave with a circle of grey nothingness floating in it. Sonic encounters Eric the Echidna, who kicks him into the circle and sends him to the Fourth Dimension. Shortly afterwards, Carrie takes Tails to the same cave and throws him into the nothingness too.