1. Make sure you have an original Japanese copy of Sonic Adventure for the Dreamcast. The Limited Edition + International versions won't work.
2. Start a new game from scratch, with no existing SA save data on your VMU.
3. Complete the first level during Sonic's story and meet tails. Immediately after the game saves when unlocking Tails, exit and start his story.
4. Complete the first two levels during Tails' story, and then go to the entrance to the parking garage in Station Square.
5. Lean to the right, and proceed to do Tails' spin attack while holding the L and R triggers.
6. If you're lucky, then Tails' model will freeze, and slide through the invisible wall, prompting the screen to flash blue.
You'll now be in an Adventure Field called "Eggtropolis", which is presumably faintly-seen in the background texture for Mystic Ruin's jungle area later on in the game. The Adventure Field is around the same size as Mystic Ruin, but it has enterable buildings which seem to have the same textures and models from the interior to the Egg Carrier. There isn't any background music, and when entering some buildings, levels from the Egg Carrier load up from Sonic's storyline, and then the game freezes.