Wunderland
Wonderland (the direct translation of the original German name "Wunderland") is a place of an unknown location, that only appeared in the Club Nintendo comic "Super Mario: Mario im Wunderland".
The only known inhabitant of Wonderland is a wizard called Crab, who lives in a castle and is said to be able to fulfill any wish. Some other species and characters, that are otherwise found in Mushroom World and other locations, are also seen in Wonderland, such as Whispy Woods, Toad, Kirby, Yoshi, Donkey Kong, Wario, Bowser and a Koopa Troopa. Additionally, during the events of "Mario im Wunderland", Fox McCloud with his spaceship lands in Wonderland and helps Mario and his allies.
In general, Wonderland appears as a very green land, covered with trees and bushes. Only Crab's castle and a confusing signpost (with directional descriptions such as "here", "there", "up", "down") are an evidence of the Wonderland's civilization.
Mario only accidentally travelled to Wonderland. Actually he was about to help a woman whose house was submerged in water, but then a whirl catched him and took him to Wonderland. Moreover, Mario's clothes changed and gave him a ladylike appearance. Finally, Crab fulfilled Mario's wish to get back home, and he also got his old clothes back.