A screenshot of Samus Aran and Yoshi fighting on the UFO in Super Smash Bros. Melee
The UFO is an unidentified flying saucer that originates from EarthBound. It is featured in the stage Fourside in Super Smash Bros. Melee and later in Super Smash Bros. Ultimate. The UFO randomly appears from the leftmost or rightmost side of Fourside, and serves as another platform for the fighters to fight on. The UFO has a very slippery surface, reducing traction for fighters. After a prolonged period of time, the UFO disappears, but returns to the stage after a period of time.
At the beginning of EarthBound, this flying saucer appears along with the ominous message, "The War Against Giygas!" Adding to the mystery are three more UFOs, which fly amok in the skies over Onett; how all of these alien craft are connected is anyone's guess. In Super Smash Bros. Melee, this slippery UFO appears on the Fourside level.