Bowser Jr.'s mass producing Bob-ombs in this underground cave, and has fortified many of its walls with blocks that can't be broken by a mere plumber. The Bob-omb is as much a valuable ally as it is an enemy. Other treacherous features of the level include drawbridges over chasms and Spinies that attack from the ceiling.
Go into the pipe to drop on a huge mushroom platform. Up ahead is a Bob-omb, a recurring enemy in the Mario series. It is vulnerable to all forms of attacks, but instead of dying instantly, it goes out with a bang after a short delay, destroying walls and blocks nearby while potentially hurting you. You must use this unique feature further to the right, as you reach a dead end formed by solid stone blocks. A pipe nearby spawns Bob-ombs continuously, so use one to destroy the wall and continue right.
To the right, you will find that much of the area is composed of blocks. A single red Koopa patrols this area, but there is another enemy you have to worry about. You might have confronted Spinies during Lakitu's aerial raid, but wild Spinies are notably more aggressive. They will drop from the ceiling once you get close, spinning toward you at high speeds. Luckily, their backs are exposed whenever they're spinning on the ground, letting you stomp them safely without worrying about their spikes.
Jump to the wide red mushroom platform. There are two Bob-ombs wandering around in an enclosed block formation, where you can potentially get a 1-Up. Knock one of the Bob-ombs down from below, letting it blow a hole in the block arrangement. Jump up and hit the leftmost block to make a 1-Up pop out, then drop down and go to the right quickly to obtain it. Jump across some more mushroom platforms and you should pass the checkpoint flag soon after. Further right, you can spot a cache of coins enclosed by blocks, as well as a Star Coin in this same arrangement. In both instances, you must place a Bob-omb beside the wall of the block box so that you can obtain its contents after the explosion. Note that blocks with an X pattern can't be blown up no matter what.
Next up, you must cross some drawbridges. They are what you must use to cross bottomless pits, but they can open pretty wide frequently. Obviously, the ideal time to cross them is when both sections of the bridge are level. On your second drawbridge, you should notice the second Star Coin inside a box of blocks up above. There is a Bob-omb inside the box as well, so it should be obvious what you must do. When the drawbridge is lifted, try to hit the Bob-omb from below, causing it to blow up, making a hole that you can jump into. Get in the block formation and get the coin.
The next few drawbridges sport common obstacles: falling Spinies. They can be difficult to evade, especially with the drawbridges' tilting. Soon after, you should come to a drawbridge with some stone blocks obstructing a warp pipe underneath. There also just happens to be a Bob-omb spawning pipe nearby. Use the Bob-omb to clear the blockage, then descend down the warp pipe. You will be brought to a bonus area with the third Star Coin surrounded by a solid stone block structure. As with before, you have to use the Bob-ombs to clear a path to the Star Coin. With only five Bob-ombs in the room, it helps to strategize a bit. The simplest way is to make a horizontal route to the coin, by blowing up the two pillars to the left or right of the coin.
After you're done, go up the right warp pipe to return back to the main level. You have only skipped one featureless drawbridge, but you're right next to the exit pipe. Watch out for the Spiny as you exit through the pipe and back to the surface. The flagpole area has the traditional stair of blocks that you can hop to the top to reach the highest section of the flag. The World 4-Castle is looking close, but you must cross one last section of the poison swamp before you get there.
- This coin is surrounded by solid blocks next to the ground. Have a Bob-omb blow the blocks up.
- The star coin is in a compartment above a bridge. Knock the Bob-omb down to blow a hole into the compartment, letting you jump up and collect the coin.
- Next to a Bob-omb spawning pipe is a blocked pipe entrance. Blow up the blockage and enter the secret area. Carefully plan explosions using the Bob-ombs inside to create a path to the coin.