Mecha Green Hill Zone is the fourth Zone of Sonic Chaos for the Sega Game Gear / Sega Master System, coming after Sleeping Egg Zone and preceding Aqua Planet Zone. In keeping with the other stages of this game, Mecha Green Hill consists of two standard Acts followed by a shorter, third Act which contains the boss. Geographically, this Zone is located on South Island (as is the rest of Sonic Chaos).
The different background as seen in the Game Gear
As demonstrated via the Bad Future levels of Sonic the Hedgehog CD, the nefarious Dr. Eggman seems to take cruel pleasure in converting natural Zones into twisted, metallic parodies of their former selves. According to the Japanese instruction manual, Mecha Green Hill Zone is a mechanised normal Green Hill Zone.
So, where once there was verdant, peaceful idyll, now steel palmtrees deploy coconut bombs and Boing-o-Bot Badniks trundle across bleak fields of synthetic grass. The ground has been metallized, and the azure skies turned sickly, polluted green (or orange in the Game Gear version). The Zone also incorporates the 'tilting bridges' from Gigalopolis Zone that the player has to sprint across fast or risk falling off. At the bottom of the Zone are pools of oil that the player will sink into unless they jump.
The music for Mecha Green Hill Zone is a remix of the music in Green Hills Zone, which itself was a remix of "Sonic - You Can Do Anything" from Sonic CD; further signifying the link between the areas.
||Mad mastermind Dr. Robotnik even has Coconut-Bots, so keep an eye on the palm trees. Look out for traps everywhere. Jump to save yourself from the deadly molten metal. Stay on top of the Tree Crawler Boss.
— US Game Gear version manual
||The Green Hill Zone has gone mechanical, and the skies are gray with pollution thanks to Dr. Robotnik. Don't let yourself sink into the sludge in the river or you won't get out. The coconuts in the trees are explosive, so if one drops near you, get out of the way!
— EU Master System version manual
||Eggman has remodeled South Island’s Green Hill into a world of machines!?… All of the flowers, trees and the soil is mechanized! That’s unforgivable, Eggman!!
— JP Game Gear version manual (translation by greenyvertekins)