World 6-5 (Super Mario Bros. 3)
From the Super Mario Wiki, the Mario encyclopedia
World 6-5 is the fifth main level and the sixth level overall in Ice Land from Super Mario Bros. 3. This level can only be beaten by flying with a Koopa Shell. It is the only level not in Dark Land with a 400-second timer.
The level starts with a Warp Pipe that sends Mario to a secret area with a Super Leaf, which contains another Warp Pipe leading to the end of the level, creating a loop. However, after the Warp Pipe, there is a ? Block with another Super Leaf and a Brick Block with a 1-Up Mushroom. There are also many Buster Beetles and Ice Blocks which can be thrown. In order to complete the level, the player must have a Super Leaf or Tanooki Suit, hold a Koopa Shell, fly to the top, and throw the Koopa Shell. Then, Mario will be able to use the Warp Pipe in order to reach the goal.