Hancock has been arguing for a long time that a variety of stone age monuments around the world stem from a common cultural background, from a priest caste that spread around the world to build those monuments, and that they must have come from a civilization we have not yet discovered that was destroyed by the end of the last glacial period.

It's been a while since I've watched any of his documentaries, so I'm not sure if my above characterization is accurate anymore (or ever was -- maybe I misremember). But while there isn't much (if any) evidence yet for his proposition, it's really not that farfetched. Sea levels went up a lot, and many possible sites of a small but relatively advanced civilization can have been buried relatively quickly and in ways that are hard to find now (e.g., in the black sea). Whatever the case, such a civilization(s) was not that advanced, and it must have been fairly small, else they'd have spread to areas that would not have been destroyed by the end of the glacial period. The biggest problem I have with his proposition is that it's really difficult to imagine a "priest" caste maintaining the necessary knowledge, ideology, secrecy (comsec and opsec), willpower, and cohesion across hundreds of (perhaps over 1,000) generations and tens of thousands of miles.