World 7 (New Super Mario Bros.)

World 7 is a New Super Mario Bros. world made up of clouds. You get there by beating Petey Piranha (boss of World 6 with a Mini Mushroom, or using the Warp Cannon in World 4. It is the first place Banzai Bill appears (in world order). The boss, a Dark Lakitu named Lakithunder, is speculated to have caused the attack on Princess Peach's castle in the intro movie.