World 6-9 (Super Mario Bros. 3)
From the Super Mario Wiki
There are two ways to complete this level. The shortcut can only be done if the player starts off with a Magic Wing or Super Leaf/Tanooki Suit and flies to the top of the ice structure on the right. At the top, the player will find a Koopa Troopa, two Ice Blocks, and a Brick Block containing a 1-Up Mushroom. If the player continues to the right, they will find the goal.
If the player does not have the item needed to fly, they will have to enter the Warp Pipe, which leads to a flooded underground cavern. The cavern is separated into two segments, both segments having an upper path with Munchers and a bottom path with underwater enemies. Both of the upper paths contain a Jump Block with a power-up in it, though the first underwater segment contains a Warp Pipe at the end that the player can only enter with a Frog Suit. Through the Warp Pipe is a secret area with coins and a large ? Block with three 1-Up Mushrooms. In the upper area of the second segment, there is a high Brick Block with a Switch Block that will transform all of the Munchers into coins. The final Warp Pipe at the end above the pipe with the Venus Fire Trap in it will take the player to the goal.