Mystery Box is an item and an area at the same time 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 such as World 1. These boxes contain mysterious levels which, when completed, releases a Star Coin and some coins to go along. The challenge usually has Mario defeat a lone enemy or a group of enemies before gaining the Star Coin 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, etc.
Once the Mystery Box is completed, it cannot be accessed for twenty four full hours. This also happens if Mario/Luigi loses a life in the area or fails to collect everything on time.
Sometimes, Mystery Boxes have golden in the inside of them. These golden areas lack enemies and usually contain goodies such as Star Medals or 1-up Mushrooms. An example is the rare one in World 1 which contains two Star Medals, fifteen Super Leaves, and a P-Wing.
Aside all the stated information, certain Mystery Boxes can also be accessed via StreetPass. These boxes via StreetPass are guaranteed to take place in a golden area.