World 7-5 (New Super Mario Bros.)
From the Super Mario Wiki
This level is filled with Bullet Bills and Bob-ombs. Unlike most World 7 levels, this level takes place on an overworld, grassland area and not in the sky. In the hidden area, there are several Mini Goombas, as well as the second Star Coin, blocked behind a pipe. It requires Mario to get the Mega Mushroom from a ? Block in the hidden area. There is a Banzai Bill Cannon launching Banzai Bills at the very end, right before the flagpole.
To get to the secret exit, Mario must blow up several blocks with Bob-ombs (or use a Mega Mushroom) to enter another hidden pipe that leads to a hidden underground area. Another pipe will take him to several hills and the Secret Exit, which will take Mario to World 7-A.