The Impossible? Maze

'''The Impossible? Maze' is the Extra Stage of World 4 that appears in Super Mario World 2: Yoshi's Island and its Game Boy Advance port Yoshi's Island: Super Mario Advance 3''. Like the other extra levels, it is unlocked by getting a perfect score on each of the world's eight levels. The level takes place in two areas of a maze-like underground sewer, connected by large round openings that function like doors. The area where Yoshi starts is dimly lit and consists of many separated rooms, while the other area is fully visible and interconnected via tunnels with flowing water. Many of these tunnels are at a downward angle and the stream won't allow Yoshi to travel back up. Red coins and Special Flowers are hidden throughout these paths. As the player navigates the maze, they will find that there is no clear way to progress until they push crates down to lower areas, allowing them to access entryways and platforms that were previously out of reach. The main enemies found in this level are Wild Piranhas and Grim Leechers. Like every other Extra Stage, this level's playable Yoshi is the green Yoshi.

This level features certain elements not seen anywhere else in the game, such as the aforementioned round openings that operate like doors, Grim Leechers, and Flashing Egg Blocks. Grim Leechers and Flashing Egg Blocks appear in new levels in Super Mario Advance 3, however. This is also the only level to feature the "Room Before Boss" music playing throughout its entirety.