Atlantis is a legendary city or island that appears in two of Sonic the Comic's Sega Superstars series. The legend originates from the stories of Plato, who believed there to be an island somewhere near Greece that held an empire over the world in ancient times. Nowadays, Atlantis is almost universally regarded as a "hoax", yet there are some who believe that the island still lies somewhere on the bottom of the sea, doomed to rest there after a "day of misfortune".
Ecco the Dolphin
The city in the Ecco the Dolphin largely follows the same legend, although there are a few major differences. The people of the island were advanced but were forced to flee when alien race The Vortex arrived, putting a plan into motion that saw them absorb most of the sea's food stocks. Not wishing this fate upon their planet, the Atlanteans seeded the dolphin race with the hope that they would eventually produce one child capable of saving its entire species. They also constructed glyphs that would aid such a creature on its arduous journey. After this final act, the humans fled their sinking city, leaving behind only the master glyphs.
Eternal Champions
Little is known about Atlantis in the Eternal Champions, although a bit of backstory is revealed in the Eternal Champions Special. Like true legend, Atlantis and the Roman Empire were at odds with each, engaging in a power struggle. Atlantis wished to join with Rome, but the Italians only wanted supremacy and to sink their rivals. To fight them off, Atlantis genetically engineered Trident, half human, half amphibian. A strong warrior, he was unfortunately killed when the Romans toppled a pillar on him. This pillar also held up the Great Dam of Atlantis - the waters came flooding in and sank the proud city to the bottom of the ocean.
Trident would be given another chance by The Eternal Champion and reluctantly assisted the other Champions in several adventures. The "gladiator" proved key in their first true battle, seemingly killing Nakano when the others had failed. However, he needs to be fully immersed in salt water once a day, a fairly major flaw.
Sonic the Hedgehog
In the main Sonic the Comic universe, the Aquatic Ruin Zone acts as a mirror to Atlantis, being the Mobius version of a sunken city. Sonic the Hedgehog remarked on this when he fell into the sea before being rescued by The Sentinel. The Labyrinth Zone sometimes has this quality as well.