This is the Cannon's Core from Sonic Adventure 2, where Shadow fights the Biolizard.
the stage like the Emerald Hill Zone stage in that you can travel offscreen by going of the side. A little stream cuts through the center of the stage. If you touch it, the current will carry you to a KO. Occasionally, the Biolizard will appear in the background. Don't worry, he's just for show.