World 6-B (New Super Mario Bros.)
From the Super Mario Wiki
World 6-B is the second and last secret level in World 6 in New Super Mario Bros.
This level takes place on an icy mountain. To start, Mario must enter the Warp Pipe on the far right, which blasts him to the top of the icy platform. Next, the player needs to grab several Ropes, with some of them moving, and swing to the next few platforms. At the first Star Coin, Mario must grab a Shell, throwing it at the coin. Soon after that, the player should be at an area with tipping purple Mushroom Platforms. When Mario blasts out of the next Warp Pipe, he should bounce off the Koopa Paratroopa to get the second Star Coin. The next area contains Piranha Plants and more purple mushrooms. The player will later see three Warp Pipes, which all blast Mario to the same area, but at a different angle. When the player sees more ropes, they must swing to the top platform and slide while ducking to reach the last Star Coin. Lastly, the player must make several jumps across the pipes to reach the flagpole.