The acoustics of a cave made for an unusual performance of a popular UW-Madison a cappella group. Eleven members of Tangled Up in Blue sang for about 50 people Sunday in the depths of Cave of the Mounds, a natural limestone cave here designated a National Natural Landmark in 1988 by the United States Department of the Interior and the National Park Service. After an 11-song concert lit by 300 votive candles and strings of Christmas lights, members of the group agreed it was the most unusual place they?ve performed.