While I was traveling around the national parks in the southern states, I ended up at the Luray Caverns. Inside there's a circular trail about 2.5km long with several levels. There were two really cool things: a lake that's literally like a mirror (second photo) and a stalactite organ.

By the way, this organ proudly holds the title of the world's largest musical instrument. A Finnish band called Pepe Deluxé even recorded a whole track for their album on it. You can listen to it here. Even though it looks like just a gimmick to attract new visitors, you can see in the video how the hammers strike the stalactites.

According to legend, a scientist named Leland Sprinkle came here on a tour with his son. The kid hit his head on a stalactite, but the father was so impressed by the amazing acoustics of the cave that he built this contraption. When you enter the room with the instrument, the music starts automatically and you get to hear it live. Pretty cool.