Probably a super advanced sea based civilisation that evolved millions of years before us. Maybe they watch us but aren't interested in making contact because we're too aggressive and primitive... Possible they're down there somewhere but don't want us to find them. Or maybe even left the planet and went elsewhere.
Cetaceans started evolving about 50 million years ago, that is a long time to develop far beyond what we have in the last 250k years or so. We know something has probably messed around with the DNA of sperm whales 30k years ago, they all share a common eve ancestor from that time from genetic studies of them. That would seem to be impossible to have happened naturally. Sperm whales (and Orcas, possibly other cetaceans too) almost certainly have a language of some kind that people are trying to figure out.
As ridiculous as it sounds, that is the most logical explanation as I see it. Or it could be a civilisation from elsewhere that has a multi million year evolutionary advantage on us. Solution to the fermi paradox. It's not something to worry about, if a civilisation that is 50m+ years old wanted to wipe us out they likely would have already and we wouldn't be able to do anything anyway.