This is 1st hand Late 80’s in sonar on a submarine doing a Med. run on way to Italy.
Can’t say how far because we took a week doing ops on way. We were over ultra-deepwater. There is a charted area with unmistakable mechanical loud noise coming from below in the middle of nothing. I was told to ignore it, we know it’s there. It is not on any chart you will see, it’s “need to know” and since we all had Top Secret it’s above that. If you talk to submariners that worked sonar and went to the Mediterranean, then docked with USS Orion sub tender. They did the same route and some of them have heard it. Whatever it was is far lower than any of our submarines of the day could go including NR-1. There was one other thing of interest, thinking about the USO Podcast. Another time about 20 years ago off of another continent there were swarms of fast moving underwater objects using propellers that would split off in different directions. Not close to supersonic, not even sure you can do that underwater. Whatever they were they were of very serious interest well above my clearance. They were not ours, and seriously doubt they belonged to other ships in the area from an unfriendly country. Have no clue what they were or where they came from. They started fairly nearby went straight then branched out. Never heard them come back.
Would love to hear other vague stories from sonar. No one else on the ship is aware of anything outside the hull but sonar hears everything.