World 6 (New Super Mario Bros.)
From the Super Mario Wiki
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World 6 is a world in the game New Super Mario Bros. This world map and most levels are made up of rocky canyons and mountain-like terrain, though some levels borrow themes and concepts from previous worlds. This world introduces the new enemies Fire Bars, Big Deep Cheeps, Deep Cheeps, and Chain Chomps. World 6 is the only world in the entire game to feature Chain Chomps. This is the only world apart from World 8 to have two Towers before the main boss of the world.
The boss of this world is Monty Tank, a Monty Mole in a tank that fires Bullet Bills at Mario or Luigi. To defeat him, the player needs to jump on his head three times when Monty's head is exposed to throw Bob-ombs at the player.