“Ringing Rocks” (20 miles east of Butte, Montana) is one of the most unusual rock formations in the country. A similar "ringing rock" formation exists in Pennsylvania. Folks from all over travel the three-mile gravel road north of I-90 just to pound on Montana's version of "Ringing Rocks" with hammers.
Nobody knows for sure exactly why the rocks ring like bells when struck with a hammer. The rocks do contain a significant amount of iron, which might be part of the explanation. However, the high iron content doesn’t explain two other strange qualities that the rocks possess . . . Not all of the rocks ring when struck, and they don’t ring when they are removed from the site, suggesting that the ringing has something to do with the way the rocks lay against one another. Larger flat ones seem to produce an especially impressive sound when struck.