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Sonic the Hedgehog 3
Hydrocity Zone

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Keep Sonic's lungs filled with air as you explore the underwater regions of the island.

— Description, Sonic the Hedgehog 3[1]

Hydrocity Zone (ハイドロシティゾーン[2] Haidoroshiti Zōn?), alternatively spelled as Hydro City Zone[2] and also simply known as Hydrocity (ハイドロシティ[3] Haidoroshiti?), is the second Zone of Sonic the Hedgehog 3, and the lock-on game Sonic the Hedgehog 3 & Knuckles. It is an area of underground waterways located on the Floating Island.[3] As with other Zones in Sonic the Hedgehog 3, Hydrocity Zone is divided into two Acts, with a boss battle at the end of them.


Hydrocity Zone is a large area of underground waterways[3] on the Floating Island. In the background, large, purple colored pillars are seen supporting the Zone. The entire temple is also made of light brown, aqua green and gray stone bricks, some of which appear polished. In Act 2, Sonic-like statues also appear.


After Knuckles activated a trap in Angel Island Zone that sent them plummeting into Hydrocity Zone, Sonic and Tails continued their adventure on the Floating Island to stop Dr. Robotnik and his relaunch of the Death Egg.

Making their way through the Zone, Sonic and Tails eventually found and destroyed the Big Shaker, one of Robotnik's robots. Later, they found Knuckles again, who activated another trap that sent the heroes into a large room where they fought Dr. Robotnik in his Egg Vortex. After defeating the doctor, Sonic and Tails were sent to Marble Garden Zone via a geyser erupting from the floor underneath them after opening the Capsule.

Much time later, Knuckles went to Hydrocity Zone after defeating an EggRobo in Angel Island Zone. There, he found and destroyed the Big Shaker, and later fought the EggRobo in the Egg Vortex. After defeating it, he was sent to Marble Garden Zone by a geyser.


Hydrocity Zone is designed with a lot of underwater sections. As such, Water Shield monitors are commonplace here. Additionally, the various underwater sections have Air Bubbles for the player to replenish their oxygen supply with before they drown. It should also be mentioned that in underwater sections, Tails cannot carry Sonic and swim at the same time if he tries to do so underwater, instantly becoming tired the moment he tries to carry Sonic. In some cases though, the player can avoid the underwater sections by simply running across their surfaces if they build up enough running speed. However, if the player slows down or does not have enough speed when crossing water pools, they will fall into the water.

The underground aqueducts here are as well a maze of Fans that blow the playable character upwards, corridors with powerful fans that blow sideways, and more. There are also gears that rotate a cable between them that the playable character can hang on to, but will fall off upon reaching one end or another if they do not jump beforehand. Some have small metal maces rotating on them, making it harder to use them. At some points, glove on a sticks might be found to allow the player pass through water tubes they would not have been able to scale otherwise. Other tubes have water pouring down into them, making them very slippery, but will help the playable character gain further speed for avoiding falling into water pools, where numerous Badniks lurk in wait, when skimming across them. Purple-tipped columns of blue cylinders also help the playable character climb up to higher areas that they would otherwise be unable to reach unless Tails airlifts Sonic and makes it up before he grows fatigued. The cylinders move up and down, so timing the scaling of them with jumping is necessary in order to proceed higher up. Furthermore, while the rotating columns can help the playable character get up to higher areas, the ones moving horizontally can risk crushing the characters if the player is not careful. There are even many slides that help the playable character's move on in the Zone.

The Zone is full of Badniks like Jawz, Mega Choppers and Pointdexters. However, in some instances, the Blastoid turrets have to be defeated to open up passageways that only crumble when the Badnik is defeated.


Act 1

The Act opens with the playable character free-falling into the Zone. If controlled during this, they can land on dry land and take a dry, upper route over going underwater and risk drowning if a Water Shield is unavailable or popped by one of the Badniks swimming in the water, which consist of Pointdexters and Mega Choppers. If taking the lower route, smash the wall to cause the water to be swept down the passageway. Along the way, the playable character will grab vertical poles at a couple of points in the corridor to hold on to against the powerful current. If the player does not jump in order to make the playable character let go of the poles, the poles will eventually break apart in the process. When the playable character is released from the poles however, they can potentially fall into the spikes pointing against the current. Once blown out of the corridor and into the main reservoir of the Zone, the playable character can either proceed along the underwater route or use the fans hidden among the spikes in some places to be blown up above the water pools to safer routes. Turbo Spiker and Bugernauts are found above the water. Regardless, the routes eventually meet up at one point where the playable character can pass through several loops and bounce off red Springs to continue further in. Hidden Springs and other paths also can be found, especially if Tails airlifts Sonic to the upper routes and away from the water if a Water Shield is not currently equipped.

After bouncing out of the room that the last Giant Ring in Act 1 can be found in, proceed to the right to reach the last Star Post in the Act. After passing this final Star Post, a loop will take the playable character to the basin where the Big Shaker appears.

Act 2

The Act opens with the playable character being pulled deeper into the Zone by a whirlpool from the basin where the Big Shaker was fought in. As they proceed, the wall on the left that Knuckles pushes to close in on them will threaten to crush the playable character if they do not keep passing through various obstacles to reach a narrow corridor that the wall is stopped at.

From there, the playable character continues through the deeper parts of the Zone, using various water tubes to navigate the underground aqueduct. Turbo Spikers, Pointdexters, Mega Choppers, and Jawz guard the area. Upon reaching the narrow corridors similar to those encountered in Act 1, larger fans will drop down and begin blowing air, pushing the playable character further down the corridor, where more grabbable poles from Act 1 appears.

Near the end of the Act, the player will encounter some cylinders that move horizontally, threatening to crush the playable character if they do not get out of the way in time. After then smashing through the last several barricades with the Spin Dash, the playable character will end up trapped on a bridge over a narrow vertical shaft. There, Knuckles appears once more and blows the bridge up, dropping the playable character into the shaft where they are blown down one last corridor by large fans into a large open room where the Egg Vortex appears as the water level lowers before the boss fight begins.


Act 1

Main article: Big Shaker

The sub-boss of Hydrocity Zone is the Big Shaker, 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 is only safe to attack this device while it is in the middle, but the player must do so quickly, as it will soon begin to rotate the pole and stir the water, spinning the player around and leaving them helpless. The player needs to hit the Big Shaker six times to destroy it.

Act 2

Main article: Egg Vortex

The boss of Hydrocity Zone is the Egg Vortex, a modified Egg Mobile with a massive screw propeller attached to its underside, which is used to swirl the water in the arena into a tall vortex. It also throws depth charge bombs from the back of the craft, which explode to create plumes of water. Eight hits are required to destroy this boss.

Giant Ring locations

Act 1

  • At the start of the Zone, as Knuckles or Tails, the player must go to the top of the starting right wall and keep going left until the first non-brick crumbling platform. They should hold left at this point to find a Ring. As Sonic, the player should take the lower path until Sonic is launched through a large blue tube. The player must then backtrack to the same crumbling platform, hold left, and hop into the Ring.
  • After the first Giant Ring, the player should lift the characters back up. They must then carry on through this path to get to a left-facing wall beyond some fans. The player can run into this wall for a Giant Ring.

Act 2

  • After the first waterslide that ends in a curve, the player must hold left and hug the left wall until the characters enter a Ring room.
  • After the second waterslide that ends in a curve, the player should position the characters in the middle of the two walls and hop on one of the rotating cylinders. They can then go to the top of the right one's path and jump up to the Ring room.

In other media

Books and comics

Archie Comics

Main article: Hydro City

Hydro City, from Knuckles the Echidna #32.

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

IDW Publishing

Main article: Hydrocity (IDW)

In the Sonic the Hedgehog comic series and its spin-offs published by IDW Publishing, Hydrocity, or Hydro City, is an area on Angel Island. its past is mostly identical to its video game counterpart's, up until after the events of Sonic Forces.

Sonic Adventures

In Sonic Adventures, Hydrocity appears as a region located on Mobius.


  • Hydrocity Zone resembles Labyrinth Zone from the original Sonic the Hedgehog.
  • In Sonic the Hedgehog 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.
  • This is the first Zone in the series to have the player run on top of the water's surface.
  • Hydrocity Zone's music is unlockable in Sonic Generations under the name of "Hydro City Zone".
  • The music for Act 1 was remixed as the music for Aquatic Relix Zone Act 2 in Sonic the Hedgehog Pocket Adventure, while the music for Act 2 was remixed as the music for Secret Plant Zone Act 2.
  • In Act 1, the surface of the water is visible in the background and changes its view angle depending how close the player is to the surface. This graphical effect is absent in Act 2 for some reason.
  • The pronounciation of "Hydrocity" has been controversial in the Sonic fan community. Some argue that it is pronounced like a single word (similar to words like velocity), while others have argued otherwise. This was referenced in IDW's Sonic the Hedgehog Free Comic Book Day 2022, where Sonic and Tails discuss about a similar matter. In June 2022, Sega made a poll on Twitter asking users on how they pronounce the word.[4]


Name Artist(s) Version Length Music track
"Hydrocity Zone (Act 1)" Sega Sonic the Hedgehog 3 & Knuckles 2:03
Sonic & Knuckles Collection "FM Synthesizer" 1:59
"General MIDI" 0:58
"Hydrocity Zone (Act 2)" Sega Sonic the Hedgehog 3 & Knuckles 1:54
Sonic & Knuckles Collection "FM Synthesizer" 1:47
"General MIDI" 0:52


  1. Sonic the Hedgehog 3 (Sega Genesis) United States instruction booklet, pg. 16.
  2. 2.0 2.1 (in Japanese) ソニックジャム オフィシャルガイド. SoftBank. 2 October 1997. p. 62. ISBN 978-4797303377.
  3. 3.0 3.1 3.2 Sonic the Hedgehog 3 (Sega Mega Drive) Japanese instruction booklet, pg. 41.
  4. Sega (11 June 2022). So... how do you pronounce Hydrocity Zone?. Twitter. Retrieved on 19 June 2022.

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