Can't get Firefly to reproduce a cave

I want to get an image of a post apocalyptic city underground.... in a cave...

Doesnt matter what prompts I enter, I can't get it close.  It always gives me a sky, some kind of distant sunrise or sunset, the moon, stars... looking out from a cave... 

This is the most recent prompt "Post apocalyptic underground city far off in the distance" and the attached image is what it comes up with.  Surely this is a bug?  I can't get it to produce any kind of cave system.
The next prompt "Underground cave system with no sunlight" - the second image is the best I could get...  Can anyone get an image of an undergound system with no natural/sunlight?