There used to be a big bluegrass festival held in Grant, Alabama, where we performed in a large cave--called, I think,Cathedral Caverns. There was a very large room where the acoustics were magnificent, for both voice and instrument. We found that we wanted to perform every acapella piece we knew. Bill Monroe actually recorded an album there in the cave. The only problem I recall was that of the water dripping on performers and audience (and, of course, the occasional bat).