In a game defined by its endless pool of player-created levels, it sounds like a bit of an oxymoron to say that there's a secret level for you to play in it. While the fly-swatting game Gnat Attack is technically more a mini-game than a level, the way it's been concealed in the game is ingenious enough that it warrants mentioning. To unlock it, you have a couple of options; the long route is to go into the level maker, then put your controller down and wait for around 15 minutes until your screen goes dark and a trio of flies begins flying around the screen. Swat the flies with your stylus, and the game will begin. Alternatively, you can grab an item, such as a '?' block or a Muncher in the level maker, then shake it around vigorously until the trio of flies appears. Again, swat them to start the mini-game. Gnat Attack will be instantly recognised by people who played Mario Paint, where the mini-game made its first appearance. If you complete Gnat Attack, then you unlock the Builder Mario outfit for Costume Mario. As of the 1.20 update, you can also play a tougher version of Gnat Attack by shaking a Muncher until the wasp-like boss Watinga spawns. Swat him to enter the special version of Gnat Attack.