|Universe Based On||Sonic the Hedgehog|
|Introduced in Game Version||Future version|
Hydrocity Zone is the third level in World 7, Mobius. Based on the second stage from Sonic the Hedgehog 3, it takes place deep underground, in a series of ancient ruins that are heavily flooded with water.
The player starts out on a sandy beach for a brief moment, before sinking beneath the sands into the ruins below. There are two main paths; one down below the water's surface, and another up in the dry part of the ruins, and the player can alternate between them and various side paths all through the level. They will have to fend against a variety of aquatic-themed badniks, as well as an assortment of hazards. Conspicuously-placed switches will cause massive geysers to smash through the floor below, lifting the player to new heights, while arrays of aquatic turbines hold the player afloat above rows of deadly spikes.
At the end of the stage, the water mostly recedes in anticipation of the boss battle against Dr. Robotnik, piloting his powerful Screw Mobile attack craft in an attempt to stop you. Defeating him will complete the stage.