World 2-A (New Super Mario Bros.)
From the Super Mario Wiki
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World 2-A is a secret level in World 2, in New Super Mario Bros.. It is the only secret level in this world. It has two exits; the normal exit opens a pipe that leads to the world's castle, and the secret exit opens up the world's cannon.
The level takes place over a stretch of water. The level features Koopa Troopas, Koopa Paratroopas, and Spike Basses. The player must traverse across many platforms and utilize several Spin Blocks to proceed through the level. By using the Spin Blocks, the player can gain access to many coins that hover in the sky. The Goal Pole for the normal exit is accessed via the use of a green pipe near the end of the stage, located directly below a yellow one.
To access the secret exit, the player need to use the final Spin block at the end of the level. With correct timing, they need to jump off the Koopa Troopa and onto a platform containing a yellow Warp Pipe on it. Mario should go into the yellow pipe, where the secret exit is found. This can also be done with Small Mario by jumping on the Koopa Troopa, and then doing a Wall Jump on the left wall. It requires precise timing, though.