World 3-Tower (New Super Mario Bros. 2)
From the Super Mario Wiki
World 3-Tower is an autoscrolling underwater tower level in World 3 of New Super Mario Bros. 2. It is the only level in the entire game to feature Skewers. This level can be unlocked by completing either World 3-3 or World 3-A. This level has two exits - the first one unlocks World 3-4 and the second one unlocks World 3- and a Green Toad House. Mario has 500 seconds to complete the level.
Mario begins on a blue platform above the water with a single Fishbone. A ? Block containing a Fire Flower can first be found, followed by a Warp Pipe in the water leading to the next area, which will begin autoscrolling upwards as soon as Mario enters the area. Two Fishbones can first be found, followed by moving walls, which will move after a set period of time. Some of these walls are spiked. More Fishbones will then appear. After all the moving walls, there are spiked pillars coming from the left and right. In between these is a safe zone where the checkpoint and a ? Block containing a Fire Flower is. After that, a Red Ring will then appear. More Fishbones and spiked pillars will then appear, followed by a green pipe. Warping through will lead Mario to a small area with a ? Block containing a Fire Flower and the boss door. Here the player needs to defeat four Reznors on a single wheel with four rectangular ? Blocks to clear the level.
Mini Mario is required to access this secret exit. Just before the checkpoint and the area with the spiked pillars, there are two small blocks which will move away to make way for Mario to pass through. Following the block on the right will bring Mario to a mini Warp Pipe. Following the path will lead Mario to the flagpole.
This exit unlocks a Green Toad House and World 3-Warp Cannon.