Deep in the Luray Caverns of Virginia, which are known as the biggest cave complex in the Eastern United States, sits the largest musical instrument in the world.

The Great Stalacpipe Organ appears at first to be a normal organ, but instead of using pipes, the organ is wired to soft rubber mallets poised to gently strike stalactites of varying lengths and thicknesses. When the keyboard is played, the entire subterranean landscape becomes a musical instrument.

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.

The Atlas Obscura Podcast is a short, daily celebration of all the world’s strange and wondrous places. Check out this episode about the Great Stalacpipe Organ.