World 3-4 (New Super Mario Bros. 2)
From the Super Mario Wiki, the Mario encyclopedia
Mario begins in front of a Wiggler. This is followed by several ? Blocks and Brick Blocks, of which one has a Super Leaf. This is followed by a floating crate on toxic water, a rectangular ? Block containing another Super Leaf above a Wiggler, a Koopa Troopa, more crates and a Red Ring. More Wigglers can be found on crates, along with the first Star Coin. This is followed by more Koopas, more crates and a Wiggler, where a ? Switch can be found on the ceiling, making coins fall out of a green pipe nearby. A red pipe leads Mario to the next area, where the checkpoint can be found. Here, the toxic water will occasionally rise and drop, which can either help or obstruct Mario. More crates can be found here, along with another Super Leaf. Here the player has to jump from crate to crate when the tide is low. This is followed by several Koopas and more crates. After that, the green exit pipe should be in sight. In the final area, there is a lone Wiggler, followed by the flagpole.
Before warping through the green pipe near the end of the level, the player should be able to see a red pipe underneath Mario for which the path is blocked by Brick Blocks when the purple toxic water lowers. The player will need to use a Shell from a nearby Koopa to break the Brick Block when the toxic water lowers. Then Mario should wait again and when the water lowers, dash and crouch through the path to access the red pipe before the water rises again. This pipe will take Mario to an area with more toxic water, a floating crate, some blocks used as platforms and the flagpole.
This exit unlocks World 3-B.