I ran across a contest today that was looking for stories about the Underworld. Nope, not hell, well you could if you wanted too, but their underworld consisted of sewers, bomb shelters, basements, anything underground. I remember in the "The Road" by Cormac McCarthy that the father found an old bomb shelter where they hid for a night. I often wondered why they didn't stay there because there was plenty of food and a comfortable place to sleep, but I suppose it's pretty hard to run from danger when you're underground.
Anyhoo, having your story take place in an underground cave of some type could give you a whole new way to look at your story and give your characters a whole 'nother set of problems to deal with. Think about it, you're dealing with the dark, perhaps a claustrophobic place, and what dangers could be hidden in the cracks and crevices? Your setting becomes an integral part of your story.
Oh, yes, here's the link to the contest if you're interested. You have to download a app in order to enter, but there's no fee.