Sonic Adventure 2: Battle Easter Egg - Glitch in the System

Go to chao world as sonic or shadow.
Face the pond.
On your left, you'll see a bunch of "cliffs" go on the second "cliff" up and face the "sky" backround. Double-jump until you are "in" the "sky".
Move forwards until you go through the 2-D "sky".

Then turn back and go back on to land.
Your chao sould be "swimming" UNDERGROUND!!