World 6 (New Super Mario Bros.)
From the Super Mario Wiki
World 6 is a world in the game New Super Mario Bros. This world is made up of rocky canyons and mountain-like terrain. This world introduces new enemies such as Fire Bars, and 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.
The boss of this world is Monty Tank.