“Clearly, the Martian overlords are preparing a base for their invasion force.”

They are already here. Their start-up base is located in Death Valley!
http://en.wikipedia.org/wiki/Sailing_stones

Video: http://www.youtube.com/watch?v=u1hoiHvOeGc

Which would be cool, come to think of it. Imagine a fleet of Frisbee-sized flying saucers swooping in on the UN building, tiny lasers firing with little phht-phht-phht sounds, their engines a high-pitched whine like a late-Cretaceous giant mosquito. Whipping in line ahead past startled security guards, banking around corners, the formation spreading out into three-layered vees in the wider corridors, like B-17s over Schweinfurt, leaving tiny pockmarks, blisters, and lurid cursing in their wake.

They burst into the General Assembly amphitheatre and gather in tight fast-moving swarms, circling high, low, and middle. Someone shouts “Squiddies!”, and the world’s leaders dive behind desks, tables and colleagues for cover. The Secretary-General raises his gavel to bang for order, but a passing saucer burns it out of his hand, then loops, lands on his head, and from its belly noiselessly extends a probe into the top of his skull.

Ban Ki Moon’s features contort, stiffen, soften, then go slack. His lips move, his larynx twitches. “Ekhkhkh-ggggg-(cough) G-g-g-r-r-e-e-e-t-ings, Earthlings.”

Pandemonium.

“[...] we’ll be saying a big hello to all intelligent life forms everywhere … and to everyone else out there, the secret is to bang the rocks together, guys.”

— The Hitchhiker’s Guide to the Galaxy