The world's largest instrument exists deep inside the Luray Caverns in the Appalachian Mountains, and it's 400 million years in the making, Scientific American reports. It's called the Great Stalacpipe Organ, but it's unlike a typical organ, which forces air through pipes to create music — this instrument rhythmically strikes the cave's stalactites to create beautiful sounds.
How it works: Large, rubber mallets sit next to the 37 stalactites. When the organist strikes a key on the instrument, the corresponding mallet strikes the stalactites, all coordinated via a hidden mechanical device that receives electrical signals from the organ. The organ's inventor sanded down 35 of the 37 stalactites to perfect their tone (two didn't need it) and the 3.5-acre cave lends natural acoustics for the songs.