Objective: Navigate the underground and find the exit.
This stage introduces underground sections and pipes. Enter the first pipe you see to descend into a cavern filled with Goombas and Koopa Troopas. Be mindful of the Piranha Plants emerging from pipes. There's a hidden Coin Block near the beginning of the underground section; jump around the ceiling to find it. As you progress, you'll encounter more ? Blocks, some containing coins, others potentially a Super Mushroom or Super Leaf.
Secret: To find the first Warp Whistle, you need to be Raccoon Mario. After exiting the first pipe and entering the underground, continue right until you see a small gap in the ceiling with a single block. Fly up through this gap. You'll find a hidden area with a White Block. Duck on this block for a few seconds to fall through the floor into a secret room containing the Warp Whistle. This whistle allows you to skip to World 2, 3, or 4.