Mushroom Hill Zone

Mushroom Hill Zone (referred to as the Mushroom Valley in the level select mode from Sonic the Hedgehog 3) is the first zone in Sonic & Knuckles (7th Zone in Sonic 3 & Knuckles). It is a lush forest notable, as its name would suggest, for giant mushrooms that can be used as various springs and platforms. The mini-boss of this stage, Hey Ho, wields an axe and chops sections of tree trunks to throw at the player. It is a large forest filled with hills and giant mushrooms.

In the first act, when playing as Sonic, Tails will airlift Sonic down from the free-fall after stopping the Death Egg's launch. Sonic then spies Knuckles closing the door to a passageway leading to a Warp Ring for Special Stage. In the complete game, Knuckles is hiding a Super Warp Ring that takes the user to the island's Hidden Palace Zone. Here all collected Chaos Emeralds' powers are halted by the Master Emerald and they are transformed into dormant Super Emeralds. From this point on, Super Warp Rings (distinguishable by the colors of the Emeralds which it flashes) transport the player to the Hidden Palace Zone for another chance at a Super Emerald. Mushroom Hill Zone contains lots of Warp Rings compared to other zones in Sonic & Knuckles, making it important to collect as many Emeralds as possible before leaving this zone if aiming to collect all seven before completing the game. When playing as Knuckles, he's seen relaxing and enjoying his playful friends when EggRobo, sent by Doctor Robotnik, appears and tries to destroy Knuckles with a well placed bomb. Knuckles gives chase, and the game proceeds often with secondary paths not available to Sonic & Tails.

The second act takes place on a desperate run through the forest to stop Robotnik from destroying it by means of a weather machine that rapidly progresses the seasons from spring, through summer and autumn and into winter. Through different sections, the color of the forest will change from green to red to tan. At the end, the player must destroy the weather machine (in the form of a satellite dish in the ground) to turn the forest back to green and trigger the boss fight.

Boss

 * Main Article: Jet Mobile

Robotnik will appear after the weather-changing satellite is destroyed. Upon this, he will fly backwards past trees that hold spikes from one end to another, which vary in height while running after the mad scientist. Occasionally, Robotnik will spray a few of the jets in the front of his machine, either one, the other, or both simultaneously. After enough hits, he'll crash into a large tree, with a small tunnel at the bottom which Sonic, Tails, or Knuckles can run through to reach the animal capsule.

Other appearances
The Zone returns in Sonic the Fighters the area set up on a giant tree stump surrounded by giant mushrooms. In story mode this is the stage Espio is fought on.

Although not explicitly stated, the entire "Sonic and Knuckles" game in Sonic Jam for the Game.com takes place in this zone.

In the Archie Sonic continuity, Espio claims this is his native zone. This later turns out to not be the case. In Sonic the Comic, the entire population of the Emerald Hill Zone was temporarily re-located to the Mushroom Hill Zone (also occasionally referred to as the Mushroom Valley) in order to protect them from Doctor Robotnik.

Mushroom Hill is back in Sonic Generations (3DS version). The level is brought back to live in 2.5 D for both Classic and Modern Sonic (as well with a nice stereoscopic 3-D effect). Classic goes through it in the summer and Modern through the fall.

Trivia

 * On the Sonic 3 Level Select Screen, there is a level called Mushroom Valley. This may have been the original title for this level.
 * In Sonic & Knuckles alone, there are exactly 7 Warp Rings in Mushroom Hill Zone (Act 1 and 2 combined) making it possible to get all emeralds before fighting the boss. In Sonic 3 & Knuckles there are 8 warp rings, since Sonic & Knuckles alone didn't have Knuckles coming out of the hidden area.
 * In Sonic Heroes, Mushroom Hill is mentioned in Frog Forest when Knuckles says "We have giant mushrooms on my island, too... but not this huge".