The Great Turquoise Zone is a region through which Sonic the Hedgehog traveled during one of his early adventures.
History
Sonic traveled through this Zone while seeking to thwart one of Dr. Eggman's schemes involving the Chaos Emeralds, only to run afoul of Knuckles the Echidna. (VG: TT)
The Great Turquoise Zone was the first of several Zones created by half a Chaos Emerald that landed on Angel Island after the original gem was shattered by Dr. Ivo Robotnik. (STT)