- For the Sonic Mania version of this stage, see Hydrocity Zone (Sonic Mania).
Hydrocity Zone is the second zone in Sonic the Hedgehog 3. This zone is completely underground, and contains the most water of any zone in the game.
The player's character will fall from the top at the start of Act 1, landing in a large blue reservoir. There are plenty of underwater routes to take so long as the player keeps their air supply topped up with air bubbles or finds a Water Shield, though there are routes above water. Throughout the zone are spinning wheels with a hand to hold the player in place that allow high-speed travel throughout the zone. This zone also features the first instance of characters being able to run on water.
Act 2 goes further into the depths of the zone, beginning with a sequence where the left wall is closing in, and the player must rush through the flooded passageways to safety. The rest of the Act is full of blue slides to roll up and down on that make this zone one of the more faster-paced water levels in the series. The player will be launched up on a water spurt to Marble Garden Zone at the end of Act 2.
||Keep Sonic's lungs filled with air as you explore the under-water regions of the island.
||Thinking we would make a high-speed stage similar the previous games, we based this stage on Chemical Plant. The map size was already at the limit of the Mega Drive's RAM with Sonic 2, but thanks to the efforts of our programmers we were able to make them even bigger. There are also routes made specifically for each character.
— Takashi Iizuka
- Turbo Spiker — A red-wheeled vehicle with a spike attached to it.
- Buggernaut — A blue mosquito that usually comes at you in groups of three.
- Jawz — A blue, flying shark robot.
- Blastoid — Your standard cannon... Sonic style.
- Mega Chopper — Red piranha that bites you. Similar to that of previous games.
- Pointdexter — A spiked pufferfish.
- As Hyper Sonic, you can travel over the top of the level and into Knuckles' area. Once you have done this, however, you are stuck unless you have debug mode activated. You can get there also as regular Sonic if you have a lightning shield, but it requires very good timing when you jump.
- If you are playing Sonic 3 you can hear music that was thought to be in Sonic & Knuckles only. You need to be in either boss, act 1 or act 2. Wait for the countdown music for drowning to start, then jump out of the water, and the Sonic & Knuckles act 1 boss music will play.
- After being swept through the tunnel into the cavern to face Robotnik in Act 2, it is possible to skip the boss by running quickly through the area to the right, where the animal capsule can be found and activated. The end of level music will play, and the points added up, but the bomb drop and blast sounds can still be heard from the boss, and the water spout will not appear to take Sonic and Tails to Marble Garden Zone. If the console is reset however, the save game will allow you to continue from Marble Garden Zone Act 1 as normal.
- When traveling around Acts 1 or 2, after rolling at speed up a 90 degree ramp transfer from floor to wall, the player character will sometime glitch inside the wall. It seems to happen more often if the character is rolling rather than running, and if the player is holding "down" on the controller during the roll.
- In Sonic 3, if you are Super Sonic while act 2 ends, Super Sonic's palette does not unload. Sonic stays yellow until the start of Marble Garden Zone. This is fixed by locking on to Sonic & Knuckles.