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 found by getting a score total of 800 for the world's other levels. The level appears to be a dim maze-like underground sewer that would most likely seem endless, unless Yoshi notices where he can roll to the crates to reach higher areas where a door is. He can push the crates through the water to also find other items, such as red coins and Special Flowers. The main enemies found in this level are Wild Piranhas and the Grim Leechers that come out of pipes. Like every other Extra Stage, this level's playable Yoshi is the green Yoshi.

This level features a lot of aspects, including pipes that operate like doors, the aforementioned Grim Leechers, and Flashing Egg Blocks, which are not featured in any other stage (barring the stages added in Super Mario Advance 3). This is also the only level to feature the "Room Before Boss" music playing throughout its entirety.