Push open the hatches in the underwater tunnels and the current will carry you along, but beware of the traps ahead! If you go without air for too long, you'll see a countdown above your character's head. When that count-down reaches zero, you lose a Life. Replenish your air supply by jumping to the surface or taking a deep breath from big air bubbles that drift up from certain blocks.
— Description from the Master System manual.
Aqua Planet Zone (アクアプラネツト Akuapuranetto?, lit. "Aqua Planet") is the fifth Zone of Sonic Chaos for the Master System and Game Gear, coming after Mecha Green Hill and preceding Electric Egg Zone. It is the game's underwater 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 with the rest of Sonic Chaos). Aqua Planet has many underwater sections. The Zone consists of crumbling, half-submerged ruins, from which ruby-tinted offshore oil rigs can be glimpsed in the background. Aqua Planet is a stormy zone too, as the roiling clouds will attest.