Why put a GPS tracker on a rock?

An enduring mystery in Geology is why and how rocks move of their own accord over a nearly flat, mud-cracked lake bed in Death Valley National Park--the Racetrack Playa. We set up the "Slithering Stones Research Initiative" in 2011 to find out.  With NPS permits in hand, we set up a weather station, and outfitted 15 rocks with GPS loggers to record when they moved, how fast they went and where they went. This past December, Jim Norris and I saw them go, and our GPS trackers recorded what they did. Now we have a paper in PLOS ONE  that tells the curious tale of the wayward rocks of Death Valley!  This is a free download thanks to my paying for open access: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0105948

We also have a couple videos that show what we saw....See: