Entering the later part of
the stage, the switch will be in clear view to you. It
would appear that below is only a pit, but going down....
You'll discover some blocks
that cover a door!
The door leads to a room
that is covered in blocks, but inhaling the star block
reveals a bomb block. Bust the bomb block open and then go
through the top door.
And through the top door
you'll enter the room with the switch!
