In Death Valley there is a dry lakebed where rocks travel on their own. They are called the sailing stones, and each one sits at the end of a long furrow carved into the cracked mud - a trail recording a journey nobody saw happen. Some trails run for hundreds of feet.
For decades, the trails were the only evidence. The rocks moved rarely, in a remote corner of the desert, in winter - and never while anyone was watching.
So scientists instrumented the playa: fifteen rocks fitted with GPS units, and a weather station. In December 2013, it finally happened while researchers were there to see it.
A shallow rain pond had frozen overnight into windowpane ice, three to six millimeters thin. As the late-morning melt broke the sheet apart, floating panels of ice tens of meters across - pushed by a breeze of just four to five meters per second - shoved the rocks across the soft mud at two to five meters per minute. Far slower than a walk, and smooth enough that from a distance you might not notice it happening at all.
Between December 2013 and January 2014, the instrumented rocks traveled as far as 224 meters. For the first time, a trail had been watched into existence: thin floating ice, and a light breeze, moving stone.