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Hidden Blocks (also called Invisible Blocks) are ? Blocks that are invisible to players unless they hit the block. Hidden Blocks first appeared after the fourth pipe in World 1-1 in Super Mario Bros., which contained a 1-Up Mushroom. Hidden Blocks did not have an appearance, as they turn into Used Blocks when hit.
Super Mario series
Super Mario 3D Land
Hidden Blocks make their debut in a 3D Mario platformer in Super Mario 3D Land. In this game, some Hidden Blocks can slightly be seen if the player has the 3D function on.
Super Mario 3D World
Super Mario RPG: Legend of the Seven Stars
In Super Mario RPG: Legend of the Seven Stars, Surprise Boxes are invisible Treasure Boxes that contain various items from Frog Coins, Flowers, powerful healing items, and items used in battle. When Mario jumps at a spot where a Surprise Box is, it first reveals itself, and jumping at the revealed Surprise Box allows Mario to collect its contents. There are 39 Surprise Boxes in the game.
Paper Mario series
In Paper Mario, the player may come across Hidden Blocks containing coins, healing items, badges, and other items, while some are also used as stepping stones to reach other areas. Such blocks can be found by using Watt's field ability. There are 47 Hidden Blocks in the game; a list of them can be found here.
Paper Mario: The Thousand-Year Door
Hidden Blocks function the same way in Paper Mario: The Thousand-Year Door. The player can use Ms. Mowz's field ability to find them. There are 15 Hidden Blocks in the game; a list of them can be found here.
Mario & Luigi series
Mario & Luigi: Superstar Saga
Hidden Blocks appear in Mario & Luigi: Superstar Saga, containing the elusive Hoo Beans. Such blocks are found by jumping around at suspicious platforms or patterns, or in seemingly "empty" dead ends. There are 42 Hidden Blocks in the game; a list of them can be found here.
Mario & Luigi: Bowser's Inside Story
Hidden Blocks appear in Mario & Luigi: Bowser's Inside Story in Peach's Castle Garden, containing the last three Attack Pieces of the Special Attack, Falling Star. They also appear in Toad Town Mall during the Mushroom Ball Derby sidequest, in which case they contain Mushroom Balls. In both cases they turn into empty blocks, but those in Toad Town Mall disappear soon after the player obtains the Mushroom Ball.
Super Mario Maker
In Super Mario Maker, Hidden Blocks were given a sprite for use in Create mode, as a translucent gray ? Block.