Not far from Virginia’s Shenandoah National Park, deep in the damp Luray Caverns, is a giant lithophone — a pipeless organ that strikes 37 stalactites with solenoid-actuated rubber mallets in order to produce 37 beautiful tones. It is the world’s largest natural musical instrument. It can be played manually or like a player piano. More via Atlas Obscura:
In order to achieve a precise musical scale, the chosen stalactites of the organ range over 3.5 acres, but due to the enclosed nature of the space, the full sound can be heard anywhere within the cavern. The organ was invented and built in 1954 by Leland Sprinkle, a mathematician and electronic scientist. It took him over three years to complete it.
From Great Big Story, this is The Organ Below The Earth.