I wonder if it sounds like whales humping or a seismic anomaly…
But, I went straight to whales as well, this could be a great option for a quiet whale watching boat.
1. https://www.nhm.ac.uk/discover/news/2022/july/underwater-noi...
2. https://www.theguardian.com/environment/2022/apr/12/ocean-of...
3. Woods Hole, 2005: https://www.whoi.edu/oceanus/feature/how-to-see-what-whales-...
Singing.