In the game Sonic Heroes 2, four archaic bases, known as eggs (obviously, as Eggman erected all four), are present and used heavily. They originate from the 8-bit games Sonic the Hedgehog (Scrambled and Crystal) and Sonic Chaos (Sleeping and Electric); the following are descriptions of the four eggs involved.
Crystal Egg Zone
Electric Egg Zone
Scrambled Egg Zone
Sleeping Egg Zone
Crystal Egg Zone is a location in the Sonic series. It is accessible by Scrambled Egg Zone only until Team Badnik crashes into it via Electric Egg Zone.
Electric Egg Zone is a location in the Sonic series. In Sonic Heroes 2, Team Badnik inhabit the base, in spite of it being a sparking mania, with rogue electrical conduits lurking. There is a section of loose tiles near the back which conceal an underground passage.
Scrambled Egg Zone is a location in the Sonic series. It is the half of Gimmick Mountain that has not been cleansed by Team Rose. They did not renovate the entire of Gimmick Mountain Zone, and so the borders have changed slightly between Gimmick Mountain Zone and Scrambled Egg Zone.