Mystery Box is an area appearing in Super Mario 3D Land. It is a purple colored box with a question mark on it.
Mystery Boxes are specific boxes found in certain world maps in the game. These boxes contain challenges which, when completed, drops a Star Medal and some coins to go along. The challenges usually has Mario defeat a lone enemy or a group of enemies before gaining the Star Medal and the additional coins. Each challenge in the Mystery Box lasts ten seconds. They may also appear in levels. Though, only coin challenges are in these boxes and thus, they lack Star Medals, except for World 8-5, where one of the Star Medals is loacted in the Mystery Box.
Once the Mystery Box is completed, it cannot be accessed for twenty hours. This also happens if Mario/Luigi loses a life in the area or fails to collect everything on time.
Sometimes, Mystery Boxes are gold in the inside. These golden areas lack enemies and usually contain items such as Star Medals or 1-Up Mushrooms. An example is the rare Mystery Box in World 1 which contains two Star Medals, fifteen Super Leaves, and a P-Wing.
Certain Mystery Boxes can also be accessed via StreetPass. These boxes via StreetPass are guaranteed to use the gold version.