Enter a tall and damp cave populated with Skeeters. Control the water level in this submerged cavern to reach normally inaccessible ledges. Vertical movement and swimming are recurring themes in this course. You will be at a constant positional disadvantage against the Skeeters, so be extra careful around them.
Go into the warp pipe. You will end up at the bottom of a flooded chamber, with a Skeeter above. While these enemies were only a mild threat back in World 3, here they are almost always found skimming the water above you, which not only makes surfacing difficult but also increases the danger of their bombs. Swim to the switch hanging from the ceiling and hit it to cause the water level to rise. Simply swim up with the current, collecting the coins in your path if you want. At the top, go through the warp pipe.
You're now in a wide open area, one of the few horizontal sections in this level. There are more Skeeters here, but at least you're out of the water, so you can defeat them easily if you wish. There are also barrels to help you cross the lake. Don't fall in though, as the other Skeeters can make it hard to get out. You can defeat all of them to make the rest of this area easier. Get to the left shore and hit the switch to make the water level rise. Swim with the tide to the platform above you, then use the barrels to get to the higher pipe on the right. If you haven't defeated all the Skeeters yet, you will likely find yourself below them as the water rises. Find an opening to surface to avoid getting hit.
Alternatively, if you have a Mini Mushroom in your pockets, you may wish to take a shortcut. Above the pipe that took you into the wide open chamber, there is also a mini warp pipe branching off from it. Entering the mini pipe takes you to a bonus area with a bunch of Micro Goombas and a block containing a Star. You can collect some coins here as well as use the Star to knock out the Goombas, giving you a potential 1-Up. Going through the normal-sized pipe in the bonus area takes you to the checkpoint. Note that you miss a Star Coin going this route, and there's also no place after the bonus area to revert back to normal size.
If you're using the normal path and went into the pipe at the high ledge, you will be transported to a vertical area with plenty of brick blocks. Hit the lone block for a power-up, then start ground pounding a path through the brick blocks. There are some coins you can get within the wall of blocks. Once you fall to the third wall of blocks, make sure to align yourself above the 4th and 5th blocks from the right. Ground pound through the blocks and you should pass a Star Coin as you drop to the bottom. The checkpoint is on the ground, and the way forward is through the pipe on the right.
You arrive at yet another submerged shaft with two Skeeters above you this time. Hit the switch as usual to make the water level rise, then swim to the top while avoiding the Skeeters. Near the ceiling is the second Star Coin. You need to hop off one of the Skeeters to reach the coin, which is easier said than done due to the awkward jump out of the water and the erratic movement of the Skeeters. To continue to the next area, go into the warp pipe directly below the highest pipe.
The gimmick of this room is mostly similar, but there is now a stone wall that splits the cave into two sections, as well as a pair of blocks that can extend when you bump into them. The purpose of these yellow blocks is to temporarily confine a Skeeter to a smaller section of water, and you can ensure your safety through strategic use of the blocks. Hit the switch in the center to make the tide rise. Swim up the right half if you want the last Star Coin. Swim up the left half to arrive at the exit pipe. Back on the surface, use the blocks to cross the gap and leap to the flagpole.
- In the cave with a lot of brick blocks, start ground pounding directly below the power-up block. After you break the last wall of blocks, you should fall right into the Star Coin.
- In the room after the checkpoint, hit the switch and swim to the top. Bounce off one of the Skeeters to collect the Star Coin in the air.
- At the penultimate room with the extending blocks, make the water level rise and swim up the right side for the Star Coin.