Towards the end of the level you will come across a white block in mid-air with a red-shelled turtle on it and 3 coins above it. Kill the turtle and stand still on the block. Crouch down by pressing arrow down and hold until you "fall through" the block. Proceed to your right, avoiding the baddies by hiding behind the greenery. Keep going to the right and you will come to a secret mushroom room containing a whistle. Use this whistle whenin the world, but not a level, and you will be transported to a warp zone. Another hint - go as far as you can in the warp zone, then save and quit. Then return to World 1 and repeat the above. After a couple of warps, you will reach world 8!