The Impossible? Maze
From the Super Mario Wiki, the Mario encyclopedia
The Impossible? Maze is the 4th extra level that appears in Super Mario World 2: Yoshi's Island and its Game Boy Advance port Yoshi's Island: Super Mario Advance 3. It is found by getting a 100 percent score in every level in the fourth world. 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 flowers. The main enemies found in this level are Wild Piranhas and the Grim Leechers that come out of pipes.
This level features a lot of aspects, including pipes that operate like doors and the aforementioned Grim Leechers, 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.
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