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 avoiding being attacked by the enemies that could cause Sonic to lose Rings. Once Sonic reaches the Goal Ring with one hundred Rings or more, the Challenge Act is completed.
The Act begins with a Sprinkler quickly appearing above Sonic, before traveling forward through the stage alone. The player must chase after it and pick up the Rings it drops before they disappear. There will be obstacles throughout the stage that must be avoided in order to hold on to the minimum amount of Rings required to beat the Act.