The cave is almost completely shrouded in darkness. When Mario hits the block, he finds an Ice Flower. It has many metallic platforms that have lanterns at both ends and alternate between which end is down and which is up. There is a secret path near the beginning that allows the player to skip all the underground parts but can only be found by a P-Acorn.
Star Coin 1: At the beginning, there are two metallic platforms on top of each other. Jump from the higher one when the right side is up to reach a ledge. Run through the path to get the first Star Coin.
Star Coin 2: After the Checkpoint Flag, you'll see metallic platforms with bricks in between. Break the bricks in the middle to reach a pipe below. Enter it. Avoid the Fire Snakes and press the P-Switch, then run right and jump onto the ledge to grab the second Star Coin.
Star Coin 3: At the end you'll encounter two Fire Bros. on standing metallic platforms. Jump to the highest platform and you'll see a pipe. Enter it and grab the third Star Coin inside.