New Super Mario Bros./World 2-4

Water is a precious resource in the desert, so it's no surprise that an underground well would be teeming with life. However, much of this small ecosystem consist of pipe-dwelling Piranha Plants, which will gladly take a bite of plumber flesh. Also present are switches that terraform the desert and the introduction of another classic Mario enemy.

Level walkthrough
As you go right from the start of the level, you might notice some blocks that are out of reach, as well as a lone switch on a thin platform. If you press the switch, you can temporarily cause the ground to shift upward, letting you access the blocks, one of which contain a power-up. After the slope is a small cache of coins that can be collected by ground pounding the blocks around them. After jumping over the first gap, the enemies become a bit more frequent, as you must contend with some Goombas and variants of Koopas. Jump over the second gap and a Star Coin should be visible in front of you. You need to use a wall jump to collect it, and do be careful of the Piranha Plant in the pipe below it.

Backtrack a bit and climb up the steps. You will now begin your descent into the underground lake. There are some Goombas and plenty of Piranha Plants in your path, one for every pipe, so move forward patiently to avoid losing a power-up or a life to these carnivorous flora. There will be some more Piranha Plants once you plunge into the lake, as well as a small island with a P-Switch on it. Press the P-Switch to spawn a bunch of silver coins in an arch over the small island. The obvious way to progress would be to climb back up, braving some more Piranha Plants during your ascent. However, observant players should notice a very small gap in the ground deep underwater. You can swim through this gap to reach a pair of pipes. The green on contains a Piranha Plant which you want to avoid, while the red one is a Warp Pipe that takes you to an underwater cavern. There is nothing more than a bunch of coins and some Cheep Cheeps, and the exit is a red Warp Pipe at the right end of the cave. You will emerge back at the surface, at a normally inaccessible section of the stage. Ignore the switch here and run to the left to get a Star Coin. To return to the main level, press the switch on the ceiling to make the ground rise up, helping you climb out of the isolated section.

As you continue to the right, you will encounter a variant of the Goomba, a Paragoomba. Like the Koopa Paratroopas, Paragoombas can fly and take an extra hit, which will knock off their wings and turn them into regular Goombas. Drop into the valley past the Paragoomba to activate the checkpoint flag. There are two blocks here, one that contains a switch while the other contains a power-up. Press the switch to cause the ground to rise, then run to the right, avoiding the Piranha Plants and the Paragoomba, then jump on to solid ground before the sand sinks back down. Past that long valley is a pair of threatening enemies that have been a part of the Koopa Troop since the very beginning. The Hammer Bros. usually fight from two layers of brick blocks, jumping between the ground and the block layers while throwing hammers at you. You can either wait for an opening and dash past them or defeat them carefully. Since there is a Star Coin where they are positioned, you will need to take the latter option. While the Hammer Bros. are vulnerable to the same attacks as most normal enemies, stomping them is inadvisable due to the arc of their hammers. Instead, you want to bump into them from below. Alternatively, you can also toss out a bunch of fireballs to hit them from a distance. After the Hammer Bros. are taken out, you only have some Koopas and a block staircase before you reach the flagpole.

Star Coins

 * 1) In a small alcove above a green pipe with a Piranha Plant inside. Wall jump up to get it.
 * 2) After emerging from the underwater cavern, ignore the switch and run left to obtain it.
 * 3) Between the two layers of blocks the Hammer Bros. fight from.

Alternate exit
This exit requires you to be Mini Mario to reach, so the hassle might not be as worthwhile as the secret exit in World 2-3, since the shortcut you unlock isn't as far. If you happen to have a Mini Mushroom in reserve or want total completion of the game, descend to the underground lake. As Mini Mario, you can walk on the water and pass through a gap at the left edge of the lake. This will take you to a mini Warp Pipe that leads to the secret red flagpole.

The secret exit opens a long path around the main route of the world that lets you skip past a few levels straight to World 2-6. You will also unlock a red Toad House on the way.