What would make the Hidden Human Hypothesis testable?

At the moment, this is a chain of possibilities rather than one supported explanation. It combines long-term isolation, a surviving population, unusual contact stories, and a possible move to the ocean. Each link needs evidence of its own.

I’d start by spelling out what observation would count against the idea, not just what could be made to fit it. For example, what could independently show that a group had lived apart for a very long time, and what would distinguish that from ordinary human history or a mistaken interpretation of an event?

There’s a very practical hurdle before the paranormal claims: a population would need a way to keep going over generations. A serious version of the idea has to explain food, water, energy, livable space, and how people could remain isolated without leaving clear traces.

That doesn’t make the premise impossible by definition, but “they’re hidden” can’t do all the explanatory work. The more specific the proposed refuge is, the more specific the evidence and practical details need to be.

The contact stories are the weakest bridge in the argument. Even if someone finds them compelling, accounts of abductions or hybridization do not by themselves show that the source is human, ancient, or underground. Those are separate conclusions being added to the reports.

The ocean idea could be an intriguing extension for a fictional setting, but as an explanation it needs its own reason to be there. A new hiding place doesn’t solve the evidence problem unless it leads to something observable that other explanations don’t account for.