I had the pleasure to take a tour of Penn's Cave in Centre Hall, Pennsylvania. (Sorry not to have a very good photo, my camera doesn't do very well in low light situations.) According to their website, Penn's Cave is America's only all-water cavern. You go through it in a boat and it was so interesting to see all of the stalagmites, stalactites, and flowstone cascades. I wish I had a close up of some of the formations, but hopefully you can get an idea of what it was like.