World 4 (New Super Mario Bros.)

New Super Mario Bros.'s World 4 is thick forest with toxic water seeping through the ground. It is a bonus world that is only unlockable if the boss of world 2 is defeated while Mario or Luigi is in mini-mario mode. Defeating the boss of this world allows Mario to progress to the snowy world 5. The world is largely inhabited by Koopas and Goombas, with Scuttlebugs dropping down from the boughs of the trees. As aforementioned a large swamp of toxic purple water flows through the world. If Mario or Luigi were to touch the water, they would die instantly. The boss of world 4 is a massive goomba that can only be defeated by using elevators to reach its head.