- For the Sonic Mania version of this stage, see Hydrocity Zone (Sonic Mania)
Hydrocity Zone is the second zone in Sonic 3. This zone is completely underground. The player's character will fall from the top at the start of act 1, and be launched up on a water spurt to Marble Garden Zone at the end of act 2.
Instruction Manual Description
||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
- 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.
Sonic statues can be seen in the background. These, just like the mural in Hidden Palace Zone, have been there supposedly even before Sonic visited the Angel Island for the first time. This opened the door to theories and speculation that Sonic's arrival may have been prophecised long before the events seen in-game.
- 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.