On the world map, go to the donut plains ghost house. You must have a feather for this to work. Enter the ghost house. Walk right until you get to a hole. Turn around and run to the left (hold b) Jump into air and you will fly because you have the feather. On the ceiling there is a gap to the left. Fly into it and land to the right of the gap. Walk until you fall down the first hole you come accross. Go through the door down the bottom of the hole and finish the level. You have unlocked a new level above the ghost house. You can get mushrooms, fire flowers and feathers and yoshis. Don't forget to save!