Chapter 4: Secret Super Mario Brothers Pipeline:

Find and use the six hidden buttons in level B of this chapter. To find the first button, when you first start destroy the black object in the wall in front of you. After you pass under a large gun and you will walk into an open area with three more black objects. Destroy the black object in the corner opposite the entrance to the area to find the second button. Then, go up the small ramp and wait until two bridges appear, on one side of another large gun. Use a charged jump from the bridge to reach a platform above the large gun to access a hidden room with the third button. Proceed through the level until reaching an area with three doors that enemies are appearing from. There should be two small doors to the right and a large door under a turret farther back (which leads to a tunnel that contains the fourth button). To find the fifth and sixth button, get to where you must use a switch to create a bridge. Do so and start crossing it. While walking across the bridge, look to the right to find an alcove with two large explosive objects in it. Destroy both of them to find the last two buttons. Note: Use the button closer to the start of the bridge first or the secret pipe will remain locked. After all six buttons are used in order, the locked door at the end of the bridge can be opened, leading to the secret Super Mario Brothers pipeline.