Experiments:

I threw some colour on to my environment thumbnails. Some of them have really gained a sense of atmosphere:

I think the ones with lighting coming up form the bottom are most effective, as it would show the way 'down' further in to hell better. I can play with bio-luminescent things such as plants and crystals to add extra light sources in the dark of the caves of Hell:

Blackreach in Skyrim is one of my favourite levels ever to exist in a game - mainly because I really love the glowing rocks and plants in it.  The fog also gives a super atmosphere and adds a sense of scale (not that it isn't a HUGE dungeon anyway). The contrast of the plants lighting the way and the man (dwarven) made structures gives great silhouettes in the dark.

Bio-luminescence in nature: