Secret Course 18
From the Super Mario Wiki
Secret Course 18 is the eighteenth secret course in the Remix 10 mode of Super Mario Run. This secret course takes place in an underground area.
The course starts off with the player dropping in from a Warp Pipe and leads into a long hallway with many Fishing Boos. There are also many floating platforms with coins on them in the area. Three of the Bonus Medals are being held by the Fishing Boos while the other two are located among the platforms. Two of the Fishing Boos have flames on their rods which players must be careful to avoid. The Warp Pipe to end the level is at the end of the hallway.