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Underwater Challenge is the fifth and final Challenge Act of Seaside Hill Act 1 in the console/PC version of Sonic Generations. It is unlocked after clearing every Stage in the Dreamcast era and is playable only as Classic Sonic.
In this Challenge Act, the player takes control of Classic Sonic in Seaside Hill Act 1. The player must here explores an underwater ruin where they need to reach the goal. To do so, they need to activate several switches along the way that will activate fans and move around terrain.