Once upon a time (in the mid 90s) I worked as a service tech for electronic equipment. One of the customer sites I went to was in a place like that south of St. Louis. It was originally some sort of quarry/mine. I had to drive about half a mile into the side of a hill. I was kind of freaked out. The customer was a music duplication service, made 1000s of copies of cassette tapes from studio master tapes. They said it was a great place, since they had no outside radio or magnetic interference to worry about, and got pure copies.