World 3-1 is a level found in the game New Super Mario Bros..
This level is an underwater level containing many Cheep Chomps.
Jump into the pipe and descend into the water. Watch out for the large Cheep Cheep enemies as Mario swims along, collecting the coins and attempting the Red Coin Ring. There is a random item box, and just beyond that, a large purple fish who will attempt to eat Mario. Fire Mario can shoot a fireball and quickly eliminate this monstrosity. Soon after there will be three pipes in a row, with only two of them spewing bubbles. Enter the pipe in the middle to reach a small bonus room with many coins and the stage's first Star Coin. When Mario returns to the main area he can obtain the second Star Coin by swimming up between the pipes. This is difficult without Fire Mario since the purple fish will be attacking the entire time. To the right there is a P button which turns all the blocks into coins. Swim quickly to reach the third Star Coin before they revert into blocks (though in box form they can easily be broken.) There is a second Red Coin Ring Mario can attempt before going up the tube to reach the flagpole.
- Star Coin 1: After the checkpoint, there should be a green pipe between a red pipe and a green pipe spewing bubbles. Mario should enter the green pipe, and he is transported to a Coin Heaven filled with coin bubbles and Cheep Cheeps. The Star Coin is at the end.
- Star Coin 2: The second Star Coin is directly after the first, in between two pipes that spew bubbles.
- Star Coin 3: After hitting the P-Switch Mario should swim all the way to the right where the final Star Coin is.