Earth
The Earth is a planet far away from the Mushroom Planet and is the setting of Mario is Missing, Mario's Time Machine, and the cancelled Super Mario's Wacky Worlds. In these games, Bowser steals important artifacts from Earth, causing chaos on the planet. As such, Mario or Luigi must retrieve these items to restore balance to the Earth.
The planet is orbited by one moon (which Mario has visited in his adventures).
Other Appearances
Official artwork for Mario Kart DS depicts various characters racing and battling on the Earth's surface, visually demonstarting the worldwide scope of the Nintendo DS' Wi-Fi capibilities.
In Mario Party 7, while all the Boards exist in the Mushroom Planet, the Mushroom Planet (when depicted in the game) resembles Earth.
Also, oddly, Wario refers to the area below the Awazon as "deep within the Earth" in the manual of Virtual Boy Wario Land. It is unknown if he is referring to the Mushroom Planet as "Earth", calling the Awazon area "Earth", or merely referring to the soil of the Awazon.
The WarioWare series indicates that Diamond City may be located on the planet Earth, as opposed to on Mario's world (the Mushroom Planet).
Appearances in other Media
In The Super Mario Bros. Super Show!, Mario and Luigi often try to find their way back to Brooklyn (their hometown in the cartoon series) usually via a Warp Pipe. In the end of these episodes, the Mario Brothers decide to stay so they can protect Princess Peach and the Mushroom Kingdom. Additionally, the live action sequences take place in Brooklyn. The first part of the Super Mario Bros. movie is also set in Brooklyn (the Mario Bros. warp to the Mushroom Planet later in the film). Additionally, in the The Adventures of Super Mario Bros. 3, many episodes featured characters using warp pipes to travel back and forth between the "Real World" (Earth) and the Mushroom Planet.
