This is probably getting into the realm of conspiracy theory now, but what better than to disguise a submarine to make it sound like a whale, and moreover convince everyone else that it is? Of course if it is actually some highly classified stealth submarine, we may never know...
It somewhat reminds me of this: https://en.wikipedia.org/wiki/Peryton_(astronomy)
And if you're trying to be all conspiratorial about it and suggest it's a submarine... well, how about making the submarine sound like a normal whale first.
Given the range over which it operates and how long it has been doing so and the fact that no one has admitted it is one of theirs suggests if it is a submarine, it is one from a major military. Those put considerable resources into making it hard to hear their submarines. This thing is way too easy to hear, which probably rules out submarines.
Seaman Jones: [teaching Beaumont] Hear it now?
Beaumont: [resigned] No.
Seaman Jones: Beaumont, at Caltech we used to do this in our sleep! You hear it now?
Beaumont: Wait a minute...
Seaman Jones: Uh oh...
Beaumont: Buried in the surface clutter...
Seaman Jones: Yeeeesssss?
Beaumont: I should go to SAPS?
Seaman Jones: Correct! Seaman Beaumont, Signal Algorithmic Processing System. Give it a week and you'll be teaching at Caltech. So, like Beethoven on the computer, you have labored to produce... [dot matrix printer rattles] ...a biologic.
Beaumont: A what?
Seaman Jones: A whale, Seaman Beaumont, a whale. A marine mammal that knows a hell of a lot more about sonar, than you do.
Schevill, William E., and William A. Watkins. “Whale and Porpoise Voices: A Phonograph Record.” WHOI unnumbered reports. Woods Hole Oceanographic Institution (contribution 1320), 1962. https://hdl.handle.net/1912/7431.
“William A. Watkins.” Woods Hole Oceanographic Institution. Accessed July 7, 2018. https://www.whoi.edu/mr/obit/viewArticle.do?id=1579&pid=1579.
I would have thought the entire ocean is covered by now?
1 - https://breakingdefense.com/2018/07/china-russia-in-the-arct...
2 - https://www.newsmax.com/philipguthrie/china-pacific-taiwan-u...
China has 7 active ballistic missile submarines. The US has 35 active nuclear attack submarines.
It's probably cheaper to run a less thorough net and then task subs to shadow any contacts made.
It was in the past. Today such things can be used for all sorts of intel tasks. Sosus is real time info about positions/speeds of all ships, surface or not. Satellites cannot do that. I imagine that the US is also using sound to monitor Chinese building activities in the south china sea. There is also, probably, some economic intel to be gained by monitoring shipping/fishing fleets, again, in a way not easily replicated from space.
You're only going to be shadowing ballistic missile submarines with attack subs, which means (total nuclear boats) - (18 Ohio class).
Based on the individual class pages (LA, Seawolf, Virginia) there are 53 active SSNs.
(Yes, I know how low the frequency is, but a good sub-woofer speaker should be able to play it audibly)