World 1-3 (Super Mario Bros. 3)
From the Super Mario Wiki
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The player starts on a long plain with some colored Semi-Solid Platforms and a Boomerang Brother. Afterward is a structure of Brick Blocks that can be broken by knocking a Koopa Troopa into them, and placed above the ? Block on the left is a hidden pink Note Block that can send the player to Coin Heaven, and the Warp Pipe at the end will bring the player back down to near the end of the level. Past the brick structure are several pillars of Hard Blocks, though the second pillar has a hidden power-up in the top block, collected by bumping into the side of the block. Afterward is a Boomerang Brother and several more Semi-Solid Platforms with Koopa Troopas and Goombas. After these platforms is the end of the level.
There is also a hidden Magic Whistle located in this stage. Near the end of the level with the Semi-Solid Platforms, the player needs to stand on the white platform and crouch. After a few seconds, the player will drop behind the scenery and can run to behind the black backdrop at the end of the level to find a hidden Toad House containing the Magic Whistle.