This level mostly takes place in an underwater area. Start by hitting the second ? block to get a Fire Flower. To get the Star Coin, swim left past the Fishbones and go in the hole. You will uncover a hidden area containing moving blocks, a couple of ? blocks, another Fishbones, and the first star coin. Then, go back to where you entered the water and exit via pipe.
From here on, the level will be auto-scrolling. Dodge the fishbones, letting them hit the walls or eachother to dispose of them. Go up as new areas open, avoiding the spikes along the way. Eventually, you'll see the second star coin to your right. Keep on going up, and you will enter an area where spiked pillars close in. The midpoint flag will be in this area, guarded by walls on the sides.
Hit the nearby ? block for a new Fire Flower. When its safe, continue going up. You will enter an area with a Red Ring. You can float along the currents from the pipes to collect the 8 coins easily. Go in the green pipe, and you will enter a small area with the third star coin. Once outside, wait until you can continue ascending, and then follow the screen up. Beware the spiked pillars that close in by moving cautiously. When it comes into sight, hide between the two small walls in the middle of the screen. Then, continue moving up until you can enter a green pipe.
Inside the green pipe is a safe zone. Grab a Fire Flower from the nearby ? block, then go into the large door to fight the midboss: four Reznors on a single wheel. You can attack them with fireballs and jump from below until the platform you're standing on begins to collapse. Then, hop onto a safe platform and continue jumping from below until they're defeated.
Secret Exit[edit]
Just before the midpoint flag, there is a mini-pipe on the right that can only be accessed under the influence of a mini mushroom. Going in here will lead to a new flagpole, unlocking a Toad House and the World 3 Cannon.