In Death Valley, rocks cross a dry lakebed on their own, each trailing a long furrow through cracked mud. Nobody had ever seen one move.
Scientists fitted fifteen rocks with GPS. In December 2013 a shallow pond froze into windowpane ice; the late-morning melt broke it into floating panels.
A light breeze pushed the panels, and the panels shoved the rocks across soft mud. Turns out the culprit was thin ice and moving air, caught at last.