Here's some real "underground music," har har! Put on your lantern helmet and repel with me down into Luray Caverns, Virginia, where an engineer named Leland Sprinkle noticed that striking the cave's rock formations produced musical tones. So, in 1954, he conceived of an organ with little hammers that strike a hollow rock when the organ's keys are depressed. It's quite musical, though with a limited sonic palette. Rather then the usual pipe organ bombast, the Stalacpipe Organ ("The World's Largest Musical Instrument!") is quiet, ghostly. The reverberating splashes of dripping water in the background sounds like sporadic electronic percussion, adding to the ambient feel. 
In 2001, United States Naval Academy chapel organist Monte Maxwell recorded a cd full of popular, classical, gospel, and American patriotic standards played on the Organ. Four-and-a-half years ago, when the album was still in print, I posted one song from it, but as it has apparently fallen into a deep cave, here's the whole dang deal:Midnight In The Caverns: Music From The Great Stalacpipe Organ
Older recordings can be heard here (the original 365 Project), and Week 15 of Tape Findings.