Hydrocity Zone

"Keep Sonic's lungs filled with air as you explore the under-water regions of the island."

- Instruction manual

Hydrocity Zone is the second zone in Sonic the Hedgehog 3 (or Sonic the Hedgehog 3 & Knuckles). This level contains the most water of any level in the game, and several parts of the level take place underwater. This is also the only level 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.

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

Act 2 takes the player further into the depths of Hydrocity, 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 being more speed based above the water's edge, with the extensive blue slides that transport the player around the level. 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.

Sub-boss
The sub-boss for this zone is Big Shaker, a spherical contraption with four rockets. This device spins back and forth through a water-filled, U-shaped pit in which you are 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 you have to be fast, as it will soon begin to rotate the pole and stir the water, spinning you around and leaving you helpless.

Although quite difficult, you 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 Big Shaker, keeping the goal sign 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.

Boss
The boss of Hydrocity Zone is the Screw Mobile. 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 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. In 3236, Knuckles the Echidna led Monk there to evade being captured by Hunter. Eventually Knuckles managed to defeat the Overlander, but not before Monk was killed. (KtE: #32)

In its first appeaerence, Hydro City strangely looks more like a sewage system than an ancient city.

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 stage 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.
 * This is the first level in the series to have the player run on top of the water surface.
 * Elements of this stage 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 has been some debate about whether or not the zone is pronounced Hydrocity (similar to the word "velocity") or Hydro City Zone. The katakana in the Japanese manual and the name of the unlocked song in Sonic Generations suggest it should be pronounced as the latter.
 * 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.