wild card: the great stalacpipe organ / by kelly heaton

This is the first ever composition for The Great Stalacpipe Organ, the largest musical instrument in the world, located deep in the caverns of Luray, Virginia. The Organ consists of hand-tuned ancient stalactites of varying sizes that are hit with solenoid-actuated rubber mallets. The mallets are controlled by a custom made pipe organ console.

Leland W. Sprinkle designed and implemented the organ during a 3-year period around 1956.  
From Wikipedia: 

"Two accounts of Sprinkle's conception of the instrument are known. From the discovery of Luray Caverns in 1878 onward, the favorable attributes of the formations for creating musical tones were well known. One of the earliest references to performances of lithophone music in Luray Caverns comes from the tour led by co-discoverer Andrew Campbell for a group sent by the Smithsonian Institution in 1880. According to a summary of the report incorporated into the earliest printed guides to Luray Caverns, Campbell surprised the group by playing a tune on a formation, probably the one that came to be known as the Organ. By the early 20th century, performances of folk tunes, hymns, and other well-known pieces were a regular part of guided tours. According to the modern guided tour, Sprinkle is said to have conceived the idea for the Great Stalacpipe Organ during one of these performances when he toured Luray Caverns on his son's birthday in 1954.

A variation of the story of the instrument's conception is that Sprinkle's son Robert hit his head on a stalactite, producing a tone that inspired Sprinkle to invent the instrument. This account is the one published in a Meccano Magazine article from 1961 and in an article in the 1959 Rosicrucian Digest."