I wonder, then, how do you know they are there?Quote
You are right about the frequencies, they are down at the microhertz levels but I can tell you that the transducers do not pick these up directly. Not even supercooled gravimeters in Antartica can get down past 2500 microhertz.
! I have listened all the way through the article (I use assistive software) and have been doing some googling about 'lines of low frequency sound energy'. I have also over the years read much about the Templars; and about Chartres Cathedral and its labyrinth. I would love to have visited there.