World 3 (New Super Mario Bros. Wii)
From the Super Mario Wiki
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World 3 (also known as the Frozen World) is the third world in New Super Mario Bros. Wii. This world is ice-themed and introduces the Penguin Suit. This is also the first and only world in the game that has a switch that can change the obstacles slightly in levels, similar to the Switch Palaces from Super Mario World. It is also the only world in the game with two parts. On the map is an area with ice that has two Ice Bros. on it. If Mario and co. are hit by them, they will endure a battle game where they will have to complete the Enemy Course. Lemmy Koopa is the boss of this world.