Earth

"Come on, let's go home. To the planet as cool and blue as me!" - Sonic, Sonic Adventure 2

Earth, also known as Terra or Sol-3, is the third planet of the Sol System, located within the Milky Way galaxy. It's covered with approximately 80% water and 20% landmass. A tilt in its axis causes the planet to experience various climate changes called seasons during its orbiting of its sun. As a result of its nitrogen-oxygen atmosphere and sufficient amount of light, Earth is the only planet in its system capable of supporting most forms of life. Its primary intelligent species are humans, thought this tends to vary depending on the dimension of choice.

Earth's exact role in the nature of the cosmos varies across the multiple dimensions, ranging from either insignificant to fairly important. It also has radically varying histories, levels of technological development and intelligent species.

Earth Prime
For the purposes of this story, Earth Prime is the home of Murdoc, Karla, Terry and the other original characters. For the most part, it's not unlike our own Earth, though the level of holographic technology is implied to be significantly higher, as evident by attempts to recreate the Yu-Gi-Oh! duel deck hologram system using little more than homemade technology in a garage.

Earth (Sonic)
Sonic hails from a version of Earth very different from the others. Here, the landscape has significantly different formations, resulting in an abundance of naturally forming loops and twists, as well as an inexplicable number of springs and boost pads. It also is home to a number of different anthropomorphic animals like echidnas, cats and foxes, as well as several deities.

Its most prominent feature is the Chaos Emeralds, seven mystical gems which can harnessed to create limitless power. These, in turn, can be controlled by the Master Emerald, a larger gem of greater power. They have brought about and brought an end to numerous civilizations over the years and generally figure prominently into Sonic's adventures.

The technology level of this Earth is implied to be generally higher than our own, specifically in the areas of robotics and space travel. It has had contact with alien species such as the Black Arms and the Babylonians as well as several advances in space travel like the space colony ARK and Eggman's Ultra-Accelerating Space Elevator.

Earth (Ashley)
Ashley's Earth is similar to our own, with only two notable differences. The first is the confirmed existence of ghosts, which can only be observed by people with certain mental clarity. The second is significant advances in the field of neuroscience, which has allowed for technology that can transfer or alter the memories of individuals.