[throwaway1507]

I don't think it's a pretty good one. It sounds crazy. What do they eat? Where is any evidence of their existence and history? Where do they make their technology? Why is there no heat signature? Why not a single one ever tried to run away and expose them, or just got lost into our world?

[gfodor]

Yeah I mean ultimately I can put forward falsifiable tests for most of these. My hypothesis is they are in a large void under one of the oceans, in physical pods. They don't eat solid food, but absorb a liquid diet. They probably do produce a heat signature, if we looked. I don't have a good explanation for the relative lack of evidence of their surface phase, but we do have some evidence that timelines around technological civilization are very off base. For your last point, I think most of them are not physically able to travel to the surface at all, and any instance of crashed manned UAPs (which imo, are no longer manned at all) may have explanations of the kind you state. But that's all been eliminated now - I would be surprised to find out we have any surface visitations by anything other than things which absolutely must be manned. (Of which I can speculate, but the null set is a possibility.)