What can move 700-pound rocks across a flat desert? If you said "Jews in ancient Egypt," that's not the answer we're looking for. In fact, the "Sailing Stones" of Death Valley are propelled by wind force across the sand, basically huge, heavy tumbleweeds. So if you are ever camping in Death Valley, or parking in your shiny SUV, beware of tumbling rocks. The picture comes as an unrequested courtesy of a fellow on Flickr. For more information on the Sailing Stones, you can also have a look at this wikipedia article.
The movements of these rocks were apparently mysterious enough for some eggheads to do a bit of research into the subject, and they concluded that it's the wind, though to this day, no one has seen the damn things move:
It is the conclusion of this research project that wind acting alone on a saturated, but not necessarily frozen playa surface is responsible for sliding rock activity. Saturation may be achieved after local precipitation events and/or as a result of ground water discharge through springs. The proliferation of cyanobacteria and the deposition of a fine clay film assist the process by establishing a remarkably low-friction surface. It is further concluded that rotating winds are likely contributors to the dislodging and incipient traction of rocks on the playa. Airflow is greatly influenced by the Racetrack’s elevation, flatness and surrounding terrain configuration, which results in a great degree of sliding activity toward the north and the northeast. However, until researchers actually observe the rocks in action, the cause still remains controversial.