I still have to see a case reasonably well-built for an "in-house" UFO "theories"; most of them are, sorry, laughable.
Lets check briefly some issues.
First of all, the killer argument. No evidence. Not a single shred. Not a single steel tool, no fossil tire, rusty turbine, nothing. Mind you, its not just the absence of fossil record for a species, its for a genus and possibly family or even higher. Whom they evolved from? Dinosaurs? Squids? Birds? Some other hominid? Not a single fossil of one of these critters being more "brainy" than expected? But wait, it gets worse! A technological civilization needs resources to grow. Where are their ancient mines? Why don't we find evidences that someone has been mining significant volumes of cooper, iron, uranium, etc. (and with advanced tech) from the deposits we are currently mining? Where are their farms? Where are their tools and buildings? Come on, these folks are proposing a civilization with advanced tech (advanced when compared to ours nowadays) in the past and pointing at stone buildings? Where's the concrete, where's the steel? Where are the tools? Not a single steel bolt!
Consider the size requirements of a technological civilization. Look around yourself. How much can you downsize it? Its not just a matter of genetic diversity (where you would try to apply the controversial 50/500 rule); its also a matter of cultural diversity and task specialization. You need people to develop all the technological and cultural aspects of a civilization and produce all the goods (food, tools, machines, clothes, furniture, books, etc.) it needs. Your chances of achieving this with a small number of specimens are dim. How much people? I don't know, but certainly not 500 or 5000. So, even if you speculate that they are the last remnants of a long-gone great civilization, you are back to square one- no evidences for this long-gone advanced civilization. Not even a spark plug (warning! I just baited you).
KotA's pet speculation- "it came from beneath the sea" doesn't make it any better. So, you are hiding your city at the deep oceanic floor... Well, it must be a pretty small city, for the ocean floor has been mapped for military (and later scientific) purposes and nothing like this has been found. Just because you can't "see" the ocean floor, it doesn't mean no one else knows what's there and whats not. Add to this the fact that the areas with higher biological productivity are within sunlight reach and your proposed hidden submarine city is left with little if any space. Oh, now you will talk about underground cities under the sea... Its just a version of the "god-in-the-gaps" argument. You are trying to find a gap big enough to hide your civilization. The gap is not there, neither is the hidden civilization. Not to mention that a civilization of say, intelligent squids, would find a bit hard, for example, to develop and build internal combustion engines under the sea…
What's left? The hollow Earth, intraterrestrials? Please, all it takes to cripple this nonsense is Newtonian physics. Of course, we can also add recent stuff such as seismology.
Consider this before raising the “they have always been here” line again.
