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
The sound at the beginning was defined as someone screwing with people. But the thumping sounds accompanied by seismic effects is likely related to tunneling or some similar