I’m old enough to remember when the motion of the Sailing Stones was still a mystery. 1/

[: Paul Whitfield/Getty Images]

The sailing stones of Death Valley were first documented in 1915 when prospector Joseph Crook noted large rocks that appeared to be trace out long paths, as if they were sliding along the desert ground. Spooky! 2/

Some wild explanations have included magnetic fields or the ~supernatural~. However, the prevailing theory in the 1900’s was that strong gusts of wind were blowing the stones while the ground was muddy, causing them to carve out the bizarre trails. 3/

Enter NASA scientist Ralph Lorenz in 2006, who was interested in comparing Death Valley conditions to those of hydrocarbon lakes on Saturn’s moon, Titan. Lorenz suspected that ice, in addition to mud, may play a role. 4/

[: Titan & false-colour of its lakes from Cassini. NASA]

Lorenz froze a rock into a thin layer of ice, then put it on a surface of wet sand. He found that it only took a small wind to get the rock sliding. In 2010 his team published a paper suggesting that the conditions of Death Valley made this a likely explanation. 5/

I first encountered this particular mystery from StumbleUpon, which correctly felt the Wikipedia page for the Sailing Stones was consistent with my interests. At the time this theory was new, and without any direct observations their motion was still a question. 6/

But finally, in the year of our lord 2014, paleobiologist Richard Norris and his cousin Jim Norris used motion-activated GPS plus time lapse photography to actually observe the motion of the stones. 7/

As suspected, the necessary conditions included ice and mud. Water filled the area deep enough for a thin layer of ice to freeze overnight. As the ice melted on sunny days, even very mild wind was enough to set the stones in motion. 8/

Perhaps the reason the sailing stones were a mystery for so long had to do with the speed. Light winds carried them only a few meters per minute, which to a distant observer might not even look like motion at all. 9/

So it’s possible that while marvelling at the long tracks and wondering what caused them, visitors were actually watching the stones move in real time, they were just too slow for them to notice. 10.

Here’s Ralph Lorenz’s 2011 paper, it’s a fun lil read: http://barnesos.net/publications/papers/2011.01.AJP.Lorenz.Racetrack.Rafts.pdf

And the Norris paper: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4146553/