“The Racetrack”

http://en.wikipedia.org/wiki/Image:Death-valley-mojave.jpg

In Death Valley, rocks move. No one’s actually seen it happen, but they leave tracks hundreds of feet long. Experts attribute the phenomenon to a combination of wind, ice, and mud, but some of the stones weigh as much as a man. One 700-pound rock disappeared altogether in May 1994. Hmm.

08/28/2014 UPDATE: The puzzle is solved! (Thanks, Dan.)