I recall a show where they were talking about the undersea network of microphones. Once used to track subs, and now used to track marine animals.
At one point the guy was playing a bunch of tracks. “That is a humpback whale off the coast of California. That one is a group of Orca’s near Washington....
Then he played one that sounded like a sci-fy alien movie - it sounded part organic, part electrical. “That one? We have no idea what it is. But it is at about 20,000 feet deep in the Indian Ocean.”
I have tried to find that video again but never can.
Your comment reminds me of the famous undersea “bloop” sound, recorded by NOAA in 1997. You can search the word in quotes for more background.