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UFOs (Unidentified Flying Objects) or Flying Saucers are the basic means of transportation for aliens. UFOs are high-tech vehicles that can travel large distances in space. There are also many different kinds and shapes some look like saucers (hence the name) and others look like squares.
Super Mario series
Super Mario Galaxy contains many flying saucers that appear mainly in Battlerock Galaxy and Dreadnought Galaxy. These UFOs have their own gravity, and Mario and Luigi can walk on the top and the bottom. A crashed UFO floats on the Lava Planet in the Melty Molten Galaxy. Additionally, in the intro cutscene, Bowser calls a single UFO to dig the Princess Peach's Castle surroundings before lifting it. The same UFO appears flying around the Peach's Castle in the cutscene traveling to Bowser's Galaxy Reactor. It is not seen being sucked into the Black Hole at the end of the game, however.
In certain levels of Super Mario 3D Land (such as world 1-3) where a pair of Binoculars are found, the player may be able to use them to spot a white UFO flying through the sky, although it eventually flies up and out of sight.
Mario Party series
A UFO appears in Mario Party 6 in the Castaway Bay board at night near the star space. They also appear in the nighttime version of the minigame Freeze Frame. In this minigame, players can take pictures of UFOs, which are worth three points.
Super Mario Land series
UFOs make a few appearances in microgames in the WarioWare games. Some examples include: Repellion and UFO Assist (WarioWare: Mega Microgame$); Alien Abduction (WarioWare: Twisted!); Wario PI and Unintelligent Life (WarioWare: Smooth Moves); and Nasalblaster (WarioWare: D.I.Y.). In Game and Wario, UFOs must be shot down in Dribble and Spitz's minigame, Taxi. Additionally, Orbulon has his own pig-shaped "UFO" called Orbulon's Oinker.
Super Smash Bros. series
The stage Fourside that appears in Super Smash Bros. Melee features a giant UFO that occasionally shows up on screen. Players can jump on top of it, although it is very slippery, and will disappear over a period of time. The UFO then reappears after some time, and players can yet again fight on top of it. On the Smashville stage that appears in Super Smash Bros. Brawl, a UFO appears at random times in the background of the stage, having no effect on the match.