This level starts off mostly dark, save for the light emanated by your character, and weak lights from ? Blocks and torches. Having Fire Flowers is recommended; they let you fight enemies from farther away while also lighting the torches found in the cave.
Lighting up these Switch Panels illuminates the starting portion of the cave, and opens a passage to the next. You’ll encounter similar rooms throughout this level.
Green Star #1
Lighting up the four torches in this room reveals the first Green Star. Watch out for the Spinies in the area!
Green Star #2
Not far from the Checkpoint Flag, you come across ? Blocks, with another Switch Panel puzzle room in the distance. Instead, continue to the right from the ? Blocks to find a hidden passage leading to the second Green Star.
Stamp
The large lava pit that you cross via narrow paths and a moving platform hides the Stamp. Instead of continuing forward, jump off the moving platform to the right once it reaches its final destination. The Stamp is very close by.
This can be a tricky Switch Panel room. A central moving platform has a Fire Piranha Plant whose attacks can be obscured by the lava’s glow, and the platforms in the corners of the room have Spinies protecting the Switch Panels.
Green Star #3
The pipe exit room after the final Switch Panel puzzle hides a secret. You can reveal an invisible block on the wall behind it, and use it to reach the final Green Star.