Mickeyjunk Mountain is a gigantic pile of miscellaneous items functioning as an artificial mountain.
Mickeyjunk Mountain was created as a side-effect of the enchantments placed by Yen Sid on Wasteland, which made discarded Disney merchandise end up there rather than be destroyed. While most of these items were put to good use as raw material throughout Wasteland, Oswald the Lucky Rabbit, to his blind hatred of Mickey Mouse, kept kicking every bit of "Mickey-junk" (as he called it) into a single pile in what was originally Wasteland's junkyard. Over the decades the pile grew to a gigantic size, becoming known as Mickeyjunk Mountain.
In the last days of the Blot Wars, Oswald and his allies hatched a plot to finally defeat the Shadow Blot, who had ravaged their realm. They took Yen Sid's magical Jug up to the summit of Mickeyjunk Mountain and lured the Blot into it, trapping the monster there for years. To keep a tighter watch on the bottle-turned-prison, Oswald made himself a home on Mickeyjunk Mountain, abandoning Dark Beauty Castle to the Mad Doctor.
Behind the scenes
Mickeyjunk Mountain was first seen in the 2010 video game Epic Mickey.
Many locations in Wasteland are based on Disneyland rides and theme areas; Mickeyjunk Mountain is no exception, being a reflection of the Matterhorn Bobsleds. The connection would have been even clearer in early plans which had Oswald first appear wearing a Yeti-like exosuit.