The organ is comprised of 37 stalactites scattered over 3.5 acres of the Luray Caverns in Virginia. When a key is pressed on a central console, a mallet strikes one of many gargantuan stalactites scattered throughout the cave, producing a specific note which resonates throughout the entirety of its 64 acres.
The instrument was conceived by Pentagon employee Leyland Sprinkle, in part to make up for his hilariously precocious name. Appropriately, Sprinkle's brainchild came about after damaging the brain of his child. In a trip to the caves in 1956, Leyland's son Robert hit his head on a stalactite, and instead of phoning the paramedics or dropping to his knees to curse a cruel and uncaring God, Sprinkle became fascinated by the sound of the vibrating rocks. He then spent the next 3 years shaving stalactites to get them perfectly in pitch, and the next 50 years presumably driving away the invading Molemen with his giant earth organ. They have notoriously sensitive ears, you see. That's Moleman Anatomy 101 right there.
"You see, kids, if the music ever stops, our surface world will surely fall beneath their savage claws."