Aqua Planet Zone

Aqua Planet Zone is the fifth zone of Sonic Chaos for the Sega Game Gear/Sega Master System, coming after Mecha Green Hill and preceeding Electric Egg Zone. It is the game's under-water zone. In keeping with the other stages of this game, Aqua Planet 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).

Why it's called Aqua Planet is something of a mystery; but it's certainly aqua, with underwater sections aplenty. The Zone consists of crumbling, half-submerged ruins, from which ruby-tinted offshore oil rigs can be glimpsed in the background (these are presumably the Doctor's work). Aqua Planet is a stormy zone too, as the roiling clouds will attest.

Boss
Throughout Sonic Chaos, and in a marked change of his tactics from other early games, our old friend Doctor Robotnik apparently thinks he's too important to face Sonic or Tails personally (until the last boss). Instead the Doctor deploys larger, tougher, nastier badniks as boss robots.

Remember those tiny, annoying, bullet-shaped birdbots from Sky High Zone in Sonic the Hedgehog 2 (8-bit)? Well, they're back. This leaping sphere of a boss starts by sending a wave of the minature badniks against Sonic / Tails, and proceeds to periodically spit out more like torpedoes for you to dodge. Once you've done enough damage to destroy the upper hemisphere, yet more of the metallic critters spew forth as missiles.