This is first hand Late 80’s in sonar on submarine doing a Mediterranean. run on way to Italy.
Can’t say how far because we took a week doing ops on way. We were
over ultra deep water. There is a charted area with unmistakable
mechanical loud noise coming from below in middle of nothing. After all these years the only thing I remember for sure is a reciprocating sound, but it was more complex. 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. Point is if you talk to submariners that worked sonar and went
to Mediterranean, then docked with USS Orion sub tender. They did same
route and some of them have heard it. No doubt about it.