World 4-3 (New Super Mario Bros.)
From the Super Mario Wiki
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This level takes place underwater, and it contains several Unagi and Mega Unagi. The player should begin by entering the pipe to go underwater. Once Mario is underwater, the level will start auto-scrolling. The player must maneuver through the water while avoiding the Unagi and Mega Unagi. The first Star Coin is in a little crevice about a quarter into the level. After the coin is a Red Ring. The second Star Coin is found about halfway into the level between two Unagi. If the player presses the ? Switch and then goes into the green pipe that is next to the yellow pipe, they will find the final Star Coin. After going through the Pipe at the end of the area, Mario will reach the Goal Pole.