World 3-2 (Super Mario Bros.: The Lost Levels)
From the Super Mario Wiki
After a cutscene showing the player character walking into a Warp Pipe, they will be brought underwater. The level has many currents that will drag the player down and several Bloopers that are placed near them, as well as Cheep Cheeps that swim in from the right of the screen and can move through structures. After the Koopa Paratroopa near the middle of the level are several large currents with obstructions that the player will need to navigate through, followed by the Warp Pipe to the Goal Pole. If the player falls off the stairs, there are two Hidden Blocks near the Goal Pole that can be used to reach the top.