Its kinda of a shortcut to the boss. You need a fire flower. When you are on the platform where a ant with a missile throwing gun kill it. There is like somthing under it like a pipe that is facing up pand takes you down. Go up. But where the ant was. Somewhere in that platform are 4 hidden blocks. Look for them. When you find them go on top of them and shoot the blocks and go up a pipe. Then look for the door with a star on top of it.