World 5 is the fifth world that Mario and Luigi venture through in New Super Mario Bros. This world appears to be ice-themed and features many snow and ice levels that Mario and Luigi venture through. This world also features many Warp Pipes in this world so that Mario and Luigi can skip many levels and get to the castle quicker. The mini-boss here is Bowser Jr. and the boss here is Petey Piranha.
When Petey Piranha is defeated as Mini Mario, Mario and Luigi will be able to go through World 7 and goes to World 6 when he is defeated in any other form.