Special Stage (Sonic Chaos), Sonic Wiki Zone

Special Stages in Sonic Chaos are a series of five extra levels. The player can enter one of these Special Stages, each with their own unique features to utilize, to obtain one of five Chaos Emeralds (the sixth is earned through normal gameplay). To enter the Special Stage, the player must be playing as Sonic the Hedgehog and collect one hundred Rings from any of the two Acts in a Zone. The screen will then flash and the player is taken to the Special Stage. Once Sonic has all the Chaos Emeralds

Special Zone (Sonic the Hedgehog 2 16-Bit) - Sonic Wiki - Neoseeker

Sonic Chaos - Mainline Sonic Games - Sonic Stadium

Sonic the Hedgehog: Triple Trouble 16-Bit (Video Game) - TV Tropes

Special Zone (Sonic the Hedgehog CD) - Sonic Wiki - Neoseeker

List of Sonic the Hedgehog characters - Wikiwand

Special Stage (Sonic the Hedgehog) (8-bit), Sonic Wiki Zone

Sonic Advance - Wikipedia

Sonic Adventure - Wikipedia

Special Stage - SRB2 Wiki