The Great Stalacpipe Organ (1956) in Luray Caverns, Virginia, is the creation of Leland W Sprinkle, an electronics engineer who helped build one of the first computers for the US Pentagon. It employs rubber-faced solenoid-operated hammers to strike selected stalactites in the spectacular 64-acre system of caverns, giving an ethereal sound, part-piano and part-pipe organ inside a vast echo chamber. Listen on YouTube to the achingly beautiful music of this unique instrument, including pieces by the inventor, who was himself an accomplished organist.