posted on Mar, 22 2011 @ 04:21 AM
At least as far as the mechanical, rhythmic rumblings like the one in the OP, I might bet my money on the underground tunnel theory.
1) Many former military officials have mentioned the development of massive boring machines for drilling underground tunnels, connecting a nationwide
or even worldwide network of underground bases.
Similar machinery was used to bore the Eurotunnel:
2) This theory seems to make the fewest assumptions without resorting to bizarre theories of natural phenomena, like "farmland windstorms" or
whatever. Additionally the US military (and others) has extensive documented experience in detonating nuclear explosions underground, which is known
to form an underground cavity.
Wikipedia: Underground Nuclear Testing
Of course, I'm no geo-engineer - there may be easier ways to bore a large underground space.
3) I would imagine that sounds arising from underground movement might have an eerie effect above ground, even seeming to come from the sky. You have
to consider that the noise isn't just coming from one underground point, like a speaker in your basement. It would be dissipated through the earth
over a wider area, and thus might seem to have more of an ambient effect.
4) DARPA has recently been interested in the development of a positioning system for underground environments as an alternative to GPS - which cannot
always penetrate to underground tunnels - tentatively known as S-BUG. It's stated objective:
2010 S-BUG Proposer's Day Event
The objective of the Sferics-based Underground Geopositioning (S-BUG) program is to develop and demonstrate a mapping and navigation system that
provides Global Positioning System (GPS) equivalent accuracy in underground environments. The program will investigate innovative approaches that
enable revolutionary and fundamental advances in the use of natural and manmade signals-of-opportunity as the basis for geopositioning.
5) Finally, the OP's handy map seems to coincide with this theory, as it actually suggests the rough location of where just such a tunnel might be
under construction. I haven't read this thread completely yet but I'd be curious to see the dates of each of those reports.
Invisible motherships notwithstanding, the underground tunnel seems like the most likely explanation to me. As for WHY someone might be boring such
tunnels, I can only speculate.