I'm sure people remember those odd groaning/screeching/trumpet sounds that were seemingly coming from the Earth in various locations around the world a few years ago.
I'm yet to hear of an explanation for them. To me, they sound very mechanical, like heavy industrial machinery and I saw a comment somewhere referring to a video that made the link between tunnel boring machines and these sounds. Specifically when TBMs breakthrough the end of tunnel. I've watched a load of videos of these breakthroughs and they don't seem particularly loud, i.e. none of the work crews appear to be wearing any ear pro.
I was wondering if anyone recalls this video making the connection and if it sheds any light on this theory. Perhaps it depends on the rock composition etc. I don't know.
Example of the sounds in this video