Located in the Cathedral is the Great Stalacpipe Organ, the world's largest musical instrument. Stalactites covering 3 1/2 acres of the surrounding caverns produce tones of symphonic quality when electronically tapped by rubber-tipped mallets. This one-of-a-kind instrument was conceived by Mr. Leland W. Sprinkle of Springfield, Virginia, a mathematician and electronics scientist at the Pentagon.

After visiting the caverns with his son and experiencing the organ-like sounds of a stalactite being tapped, Mr. Sprinkle submitted a complex plan for a stalactite-tapping instrument. It took 36 years of frustrating research, design and experimentation to bring his dream to its present state of perfection. Three years alone were spent searching the vast chambers of the caverns to select and carefully sand stalactites to precisely match the musical scale. Only two stalactites were found to be in tune naturally.

The four-keyboard console of The Great Stalacpipe Organ was constructed by the Klann Organ Supply Company of Waynesboro, Virginia, to meet the peculiar needs of this subterranean installation. Then the organ was connected to various stalactites with over five miles of wiring.

Today, the organ plays a variety of songs, many chosen for their range and deep, resonate tones. Visitors stand enthralled as haunting melody and chords reverberate throught the vaulted ceilings. The songs, which are played by an automated system, change seasonally. The organ can also be played manually from the console, as Leland Sprinkle did for many years.