Many are now asking this same question. You can find some answers in the book at this site:
CAVERNS, CAULDRONS, AND CONCEALED CREATURES:
http://www.hiddenmysteries.com/redir/index111.html
And at this website:
SUBTERRANEAN MYTHS AND MYSTERIES
http://skywebsite.com/hollow/CCCC
This material is based on over 20 years of research.
–Wm. M. Mott
Are you suggesting that much of what is foisted onto modern society as being space-aliens or inter-dimensional visits are actually surface adventures by an advanced sub-surface species? This seems a little far-fetched to promote without presenting some firm evidence; especially on a near-skeptic site such as the SDMB. 
You say in your hiddenmysteries.com site:
Unlike skeptics, I **am **willing to accept “very strong circumstantial evidence” as well as “anecdotal evidence” for a straightforward reason: We, here in the U.S., nearly-routinely execute people on less sturdy evidence than either.
In fact, we have just gone to war on supposed evidence which, it has been admitted, is purely anecdotal and is very likely less than “strong circumstantial”–supposed evidence which has been declared classified. 
Still, I would like to see at least a little of your evidence–even though that request exceeds that which the CIA, DoD, and C-in-C have provided while, without an Act of Congress and without any Congressional Debate whether in public or in secret, committing the U.S. to War.
> Are you suggesting that much of what is foisted onto modern society as being
> space-aliens or inter-dimensional visits are actually surface adventures by an
> advanced sub-surface species? This seems a little far-fetched to promote
> without presenting some firm evidence; especially on a near-skeptic site such
> as the SDMB.
>
Yes. This is not really “new” knowledge, but in fact is very ancient.
I recommend that you listen to the radio interview in mp3 format at the second URL posted.
Occam’s Razor is very clear: the simplest explanation is usually the correct one. What are the odds a vast variety of bipedal, vertebrate, one-headed, two-eyed, reptilian or mammalian-appearing specie coming to this tiny planet on the edge of our galaxy, from a dozens or scores or hundreds of worlds?
Please note that most or ALL of the “forms” described tend to fit perfectly within an earthly vertebrate template. Some of
them probably do exist, others might, many probably do not. But
the ones that do exist are more likely to be ancient earthly forms
of life than they are to be representatives of seventy different
worlds (who all also just happen to conform to an earthly
vertebrate–mammalian or reptilian/amphibian–template), just
happening to all end up at little old planet Earth, out on a most
distant tendril of a spiral arm of the Milky Way Galaxy, in a
universe which is probably teeming with variegated life.
It simply is NOT logical to assume that all of “them”–maybe any
of them–originate outside of this solar system, perhaps off this
planet. The fact that some may have achieved space flight is
only a testament to their advanced age as civilizations, their
technological superiority, and so on. Their origins, however,
could still have been from the good old bipedal, two-armed,
two-eyed, one-headed vertebrate-spawning biosphere of our
own blue planet.
It is more to the advantage of someone who is intruding or taking
advantage of someone else, to have that someone else believe that
they come from “far away,” another world, etc., rather than from the same biosphere, right under their very
noses. Or feet. Or oceans.
These contentions are supported by the traditions of every religion and mythological cycle, if one digs deeply enough.
>
> Still, I would like to see at least a little of your evidence–even though
> that request exceeds that which the CIA, DoD, and C-in-C have provided while,
> without an Act of Congress and without any Congressional Debate whether in
> public or in secret, committing the U.S. to War.
Evidence is presented in the book!
Best,
–Michael Mott
The trouble with these hydrogen-using bacteria is that hydrogen is much less effective an electron acceptor than oxygen is. An ecosystem based on chemosynthesis is going to be much less energetic. All be bacteria are going to be anaerobic. Aerobic respiration is about 11 times more efficient than anaerobic respiration, that’s why almost all bacteria on the surface are aerobic, and all multicellular organisms (and all plants and all protozoa) are aerobic.
Now, deep sea vent ecosystems are based on chemosynthesis. Bacteria use the energy from vents of boiling hot H2S, and multicellular organisms feed off the bacteria. But these are very small, very localized, and all the multicellular organisms in these ecosystems are closely related to organisms from outside…they are standard crabs, tubeworms, etc.
If there are multicellular organisms existing deep in the crust, I would bet that they will be found to be closely related surface organisms. And that they will all be very very small. These deep bacteria don’t live in caverns, they live in microscopic crevices and pores in the rocks.
The concern that I get in this is if there is enough space for a complete ecosystem to evolve to a point where they are life forms larger than bacteria.
Though not absolutely necessary, I would suppose that a certain amount of area is needed to evolve a somewhat sentient lifeform. I just don’t think even a vast network of caves has the total surface area to produce the necessary conditions for evolving a higher life form.
Yes, it could happen, but we had a whole planet with all of its different events and factors to produce us and all other living creatures. I don’t expect a similar scenario to happen in a small cave by comparison.
There would seem to be enormous survival and evolutionary advantages to having a metabolism which eats/consumes rock.
For one thing, the caves/caverns would never get too small–you would just eat them to an appropriate size again.
The main problems I see is those mentioned by Lemur866: What could such a metabolism be? Would it allow sufficient energy to evolve into larger species, and allow those species to consume their living areas, deep caverns, into ever-larger and more productive regions?
We believe that there are large sub-surface caverns on some other bodies, such as Europa and, perhaps, Mars. It is believed that such caverns might be filled with water or some other fluid, but who is to say that species living in water couldn’t be as intelligent and intellectually productive as those of us who live in gas/air? 