Hey dandelO,
This will sound a little like I have been drinking since I cant remember the names of the things I will try to post about... But I remember from some films about caving that some caves have a type of rock that glow. Perhaps there are more than one kind of rock that does this? I saw a movie called the decent, a monster movie that takes place in a giant cave system, they used the idea of the rocks (Im trying to remember the name of) as a lighting source.
Not to long ago you helped me in a thread by telling me I could use terrain as a light source. Perhaps you can find the name and a photo of these glowing rocks and use them to create an other-wordly cave, or just light it in an interesting way.
Just an idea.