The Impossible? Maze

'''The Impossible? Maze' is the fourth extra level of Super Mario World 2: Yoshi's Island and its Game Boy Advance remake, Super Mario Advance 3: Yoshi's Island''. It is found by getting 100 percent in every level of World 4. The level is a maze-like underground sewer that would most likely seem endless, unless the player can notice where they can roll the crates to reach higher areas where a door is. They can push the crates through the water to also find other items, such as red coins and flowers. The main enemies found in the level are Piranha Plants, but the biggest danger of the level are the Grim Leechers that reside in the pipes.

This level features a number of unique aspects, including pipes that operate like doors, the aforementioned Grim Leechers, which are not featured in any other stage (barring the stages added in Super Mario Advance 3), and it is also the only level to feature the "Room Before Boss" song (which plays non-stop) without a boss battle.