From the Luray website:
"Located in the Cathedral, the Great Stalacpipe Organ is 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 most unique, one-of-a-kind instrument was invented in 1954 by Mr. Leland W. Sprinkle of Springfield, Virginia, a mathematician and electronic scientist at the Pentagon. He began his monumental 3 year project by searching the vast chambers of the caverns selecting stalactites to precisely match a musical scale. Electronic mallets were wired throughout the caverns and connected to a large four-manual console. When a key is depressed, a tone occurs as the rubber-tipped plunger strikes the stalactite tuned to concert pitch.
Today, the organ is played by activating an automated system which works in a manner similar to a child's music box. The organ is also fully capable of being played manually from the console, as Leland Sprinkle did for many years."
In one of the larger rooms, a man had built an organ system. The world's only true "Rock Music". The man went throughout the cavern with a tuning fork, checking rock formations for their musical properties.The ones he selected (throughout the entire cave complex, not just this room), were fitted with a tiny selonoid rubber hammer and a sound pickup (like on an electric guitar). When the keyboard is played, or the player-piano function is turned on, the sounds are generated and amplified and played in this room. The music was quite beautiful, and different than any I have heard. The room is open for rental for functions and they told us they performed many weddings deep down underground, here!
The next picture also includes a link so you can download a sound file and hear this unusual music.