Pay a visit to Ringing Rocks state park in Upper Black Eddy, Pennsylvania during its busier months and you’ll be greeted by bell-like sounds reverberating through the air. The noise is the result of the park’s sonorous boulders, which can only be found in a handful of places on earth. When struck with a hammer, the rocks produce a clear metallic tone similar to that of a bell.

It’s not exactly a conventional space to write music, and that’s what attracted the band Square Peg Round Hole to the area. The instrumental percussionist trio recently went to the park’s 7-acre boulder field to perform a song using nothing but hammers and the rocks in front of them. In their description of the video, the band writes, "When we discovered Ringing Rocks Park, we knew that it would be an inspiring place for us to write and play music. As percussionists, we are always searching for unorthodox sound sources, and are often finding new instruments in unexpected places.” You can listen to the final result in the video above.

This isn’t the first musical performance the ringing rocks have inspired. In 1890, Dr. J.J. Ott collected a number of stones from the site and played them for the Buckwampum historical society. (It was a "rock concert" in the truest sense of the term.)

Why exactly these sonorous or lithophonic rocks ring remains a bit of a mystery. A geologist named Richard Faas sampled a few of the stones back in 1965 and took them back to his lab to better understand their musical properties. He found that when the rocks were struck they produced tones lower than what the human ear can perceive, and the audible frequencies were created by the interactions of these different low tones. This means the boulders can only function as musical instruments when accompanied by other rocks of their kind.

Geologists still haven’t agreed on the physical mechanism that gives the rocks their unique abilities, but some suspect it has to do with the freeze-thaw cycle that helped create them thousands of years ago. Other less scientifically-sound theories include radioactivity, abnormal magnetic fields, and supernatural sources.