This post is from one of several previous incarnations of this site and probably doesn’t quite fit the current format. In a former life this was a group blog and a tumblelog before it became a static jekyll site. If anything looks broken or is worded oddly that could be why. Pardon the dust.
Swirlvision have some nice screens of the hidden level in Sonic Adventure 2 for DC. Collect all 180 emblems and you can visit a fully realised 3D version of the Green Hills zone from the original Genesis/Megadrive version of Sonic..