We took a daytrip to Devetaki Cave the other day. It was overcast and I barely caught a single sunbeam inside, but the place was still as splendid as they said it would be. We didn't see any bats, but there were plenty of sparrows, and I even did the flare trick with my kit lens at f/22. I also recommend bringing a tripod if you shoot in such light conditions - some of the handheld shots here are the slightest bit shaky.
Oh, and the concrete bridge you see in the beginning? It was built for the filming of Expendables 2, thanks to which the bat population was disturbed (and thankfully, recovered later), so yeah, Hollywood sucks big time.
Enjoy, and if you ever happen to travel around Bulgaria, you must see this cave. It's really special!
P.S. I like the fact that there were other people there, because their small figures help give my photos a great sense of scale.