Hydrocity Zone

Hydrocity Zone is the second Zone of Sonic the Hedgehog 3 (or Sonic the Hedgehog 3 & Knuckles). This Zone contains the most water of any Zone in the game, and several parts of the Zone take place underwater. This is also the only Zone in the game to feature air bubbles (apart from the balloons in Carnival Night Zone) which give the player more time underwater. A Water Shield will prevent the player from drowning indefinitely, which makes it more or less mandatory in this Zone.

Act 1 takes place in a large blue underground stone built reservoir, spacious above as it is deep. Most of the Zone's underwater sections will take place here, and routes above water are possible.

Act 2 takes the player further into the depths of Hydrocity Zone, an underground water park underneath the stone reservoir above. At the beginning of Act 2, the left wall begins to close in, and the player must rush through the flooded passageways to avoid getting crushed. Another significant feature of this part of Hydrocity Zone is being more speed-based above the water's edge, with the extensive blue slides that transport the player around the Zone. Some of these slides are preceded by a spinning wheel with a hand to hold the player in place, allowing high-speed travel along them.

Story
After falling from the bridge Knuckles destroyed in the previous Zone, Sonic and Tails fall into the Zone (Knuckles jumps down instead). Upon reaching the end of the Zone, opening the Capsule triggers a torrent of water sending the player into the next Zone.

Act 1

 * At the start of the Zone, as Knuckles or Tails, go to the top of the starting right wall and keep going left until the first non-brick crumbling platform. Hold left at this point to find a Ring. As Sonic, take the lower path until he get launched through a large blue tube. Backtrack to the same crumbling platform, hold left and hop into the Ring.
 * After the first Giant Ring, lift the characters back up. Carry on through this path to get to a left-facing wall after some fans. Run into this wall for a Giant Ring.

Act 2

 * After the first waterslide that ends in a curve, hold left and hug the left wall until the characters hit a Ring room.
 * After the second waterslide that ends in a curve, position the characters in the middle of the two walls and hop on one of the rotating cylinders. Go to the top of the right one's path then jump up to the Ring room.

Act 1
The sub-boss of this Zone is Doctor Robotnik’s Whirlpool Machine, a spherical contraption with four rockets. This device spins back and forth through a water-filled, U-shaped pit in which the player is trapped, before settling on a pole in the middle of the arena. It's only safe to attack the device while it's in the middle, but the player has to be fast, as it will soon begin to rotate the pole and stir the water, spinning the player around and leaving him/her helpless.

Although quite difficult, the player can Spin Dash up the side and attack the machine while it is off-screen without getting hurt. Though the boss itself is relatively simple to beat, the danger of drowning is still present.

Although the 30-second time limit for staying underwater is still true for this sub-boss, it is impossible to drown whilst the score is being tallied. This can be tested by, after defeating the Whirlpool Machine, keeping the Goal Plate from hitting the ground by jumping into it, whilst keeping the character below the water. Eventually, the drowning music will play, but if the goal sign then touches the ground, the music will fade away and the score-tally sequence will start. The warning sound is still heard every five seconds, but the player will not drown. The timer is reset at the start of Act 2 as the player gets sucked into the passage below.

Act 2
The boss of Hydrocity Zone is Doctor Robotnik’s Waterspout and Depth Charge Machine. It has a massive screw propeller attached to its underside, which Dr. Robotnik uses to swirl the water in the arena into a tall vortex. He also throws grenades from the back of the craft, which explode to create plumes of water. Eight hits are required to defeat him.

Archie Comics
In the Sonic the Hedgehog comic series and its spin-offs published by Archie Comics, the Hydrocity Zone is known as Hydro City. It is an ancient and abandoned underground city located underneath Angel Island and is filled with water. It was supposedly constructed before Echidnaopolis, hundreds of years ago, before the beginnings of Echidnaopolis.

Trivia

 * In the first Act, statues resembling Sonic can be seen in the upper background.
 * Some of the emblems on the walls of this Zone closely resemble those seen in the Labyrinth Zone in Sonic 1, which is also a primarily water-based level. Coincidentally, Labyrinth Zone was planned to be the second Zone in Sonic 1.
 * In Sonic 3 & Knuckles, the rather unhelpful yellow spring that bounces the player back in the section with the moving wall has been replaced with an extra life. It is likely the first noticeable difference when comparing the standalone and lock-on versions, although there are many subtle differences in other level layouts (particularly Launch Base Zone).
 * This is the earliest Zone by which all seven Chaos Emeralds can be obtained in one playthough.
 * This is the first Zone in the series to have the player run on top of the water surface.
 * Elements of this Zone appear in Act 1 of Seaside Hill and Water Palace in Sonic Generations.
 * Hydrocity Zone's music is also unlockable in Sonic Generations, under the name "Hydro City Zone".
 * There is a glitch during the Hydrocity Zone boss fights where waiting underwater without a Water Shield and reaching for air when the drowning theme plays will switch to the Act 1 boss theme heard in Sonic & Knuckles. This doesn't make a difference in Act 1 when locked-on, but it still happens in Act 2.
 * In Act 2, with perfect timing, the player can jump through the slides through a curve.

Music
Act 1= Act 2= Generations=