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For the Zone in Sonic Mania, see Hydrocity Zone (Sonic Mania).

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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 under-water regions of the island.

— United States instruction booklet, Sonic the Hedgehog 3[1]

Hydrocity Zone (ハイドロシティ[2] Haidoroshiti?, lit. "Hydrocity") of Sonic the Hedgehog 3, and its lock-on version Sonic the Hedgehog 3 & Knuckles. It is a an underwater temple system located on the Floating Island. 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, underwater ancient temple system 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 Whirpool Machine, one of Robotnik's robots. Later, they found Knuckles again, who activated a trap that sent the heroes into a large room where they fought Dr. Robotnik in his Waterspout and Depth Charge Machine. After defeating the doctor, Sonic and Tails were sent to Marble Garden Zone via a geyser erupting from the floor underneath them.

Much time later, Knuckles went to Hydrocity Zone after defeating an EggRobo in Angel Island Zone. There, he found and destroyed the Whirpool Machine, and later fought the EggRobo in the Waterspout and Depth Charge Machine. After defeating it, he was sent to Marble Garden Zone by a geyser.



Act 1

Act 2


Act 1

The sub-boss of Hydrocity Zone is the 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 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 Whirpool Machine six times to destroy it.

Act 2

The boss of Hydrocity Zone is the Waterspout and Depth Charge Machine, 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

Archie Comics

Main article: Hydro City

Hydro City in Archie Comics.

In the Sonic the Hedgehog comic series and its spin-offs published by Archie Comics, Hydrocity Zone (known as Hydro City) 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.

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.


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 Mega Drive) United States instruction manual, pg. 16.
  2. Sonic the Hedgehog 3 (Sega Mega Drive) Japanese instruction manual, pg. 41.

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