Underwater Ring Machine is the fifth and final Challenge Act of Chemical Plant 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.
In this Challenge Act, Sonic must traverse Chemical Plant Act 2 and reach the goal with at least one hundred Rings. In order to obtain one hundred Rings, Sonic must follow a Sprinkler throughout the stage, collecting the Rings it drops and avoid being attacked by the enemies that could cause him to lose a quantity of Rings. Once Sonic reaches the Goal Ring with one hundred Rings or more, the Challenge Act is completed, and the player is ranked according to his or her time.