World 3-1 (New Super Mario Bros.)
From the Super Mario Wiki
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This level is an underwater level containing several Cheep Chomps. This level starts out with the player standing on a small section of land. Next to this land is a shallow pool of water that has a Warp Pipe inside of it. The player must go down this pipe in order to continue forth in the level. When Mario exits the Pipe, he will be completely submerged in water. At the beginning of the area, there will be a Big Cheep Cheep and three ? Blocks. All three ? Blocks contain a coin. Five Cheep Cheeps will be encountered shortly thereafter, followed by two Brick Blocks and a ? Block. After a Red Ring is found, several more schools of Cheep Cheeps will be encountered, making it more difficult for Mario to collect all eight Red Coins. Past an abyss, several more coins, the checkpoint, and a Roulette Block can be found. A Cheep Chomp can be encountered past a green Warp Pipe that is spewing bubbles. It will try to eat Mario as he swims through a maze of pipes that spew bubbles. A Star Coin can be found in between two of the pipes. An upside-down P-Switch can be found on a block, and by pressing it, all nearby Brick Blocks will turn into coins, allowing access to another Star Coin. After the Star Coin, a second Red Ring can be found. Past the Red Ring, a Warp Pipe can be found. The Warp Piper allows access to ground level, where the Goal Pole is located.