If you just use two layers, you can put whatever tile you want on the bottom and use cave masking tiles on the top. That way you can make a cave with any tiles. Although I'm not sure whether the masking tiles are still in SuperTux as there were some major changes to worldmap tiles. Either way, you can always use your own tilesets.
How to make worlmap like that? (Forest cave worldmap)
If you just use two layers, you can put whatever tile you want on the bottom and use cave masking tiles on the top. That way you can make a cave with any tiles. Although I'm not sure whether the masking tiles are still in SuperTux as there were some major changes to worldmap tiles. Either way, you can always use your own tilesets.