Look Out Below! is the first Challenge Act of Green Hill Act 2 in the console/PC version of Sonic Generations. It is unlocked after clearing every Stage in the classic era and is playable only as Modern Sonic.
This act has no supporting characters and is played completely in 2D with Modern Sonic. The player is put into a certain section of Green Hill Act 2 that they have to traverse in a set time limit. The catch, however, is that practically all platforms crumble once Sonic steps on them, forcing the player to make fast and quick decisions. However, if the player fails to do so, there will be Springs that will allow the player to return to higher ground. The platforms will also reappear after a while, though such delays may cause the player to run out of time.