Revealed: The mystery of California’s sailing stones 

You must have heard of the old idiom ‘As immovable as stone’. But there are stones that not only move on their own but also leave behind trails and they exist in California! The Racetrack Playa, a dry lakebed (aka playa) in the desolate, barren and ominous Death Valley of California, harbours these sailing stones. They appear to move on their own while no one is looking! Isn’t that a bit eerie?  Let's unfold the mystery behind these stones of California. 

The curious phenomenon of sailing stones 
Under the scorching desert sun, the fractured floor of the playa appears almost prehistoric in the summer. In the winter, sheets of ice and snowflakes cover it. What you get to view is dozens of stones strewn across the landscape, followed by a trail resembling the contrail of an airplane. 

Made of a complex mineral known as dolomite, these stones even alter the width and pattern of their path over time. All the stones in Death Valley don't roll around. The stones that move do so once every two to three years. The strangest thing is that all stones in the valley don't move simultaneously or in the same direction. They travel ten to hundreds of feet far, and some even make sharp 90-degree bends. The rocks range in size from a computer mouse to as large as a microwave oven (15 to 45 cm)! The strangest aspect of these stones is that no one has ever seen the sailing stones of Death Valley move. So how did the tracks show up on the playa's dry lakebed? Read on to know more. 

What is causing the stones to sail? 
Research on the sailing stones of Racetrack Playa began in 1900. Several hypotheses were put out to justify this intriguing phenomenon. 

Aliens or Magnetism? Initially, unidentified magnetic influences have been a popular theory behind the sailing stones. However, this hypothesis was disproved due to the lack of magnetic metals in the stones. Some theories claimed that aliens were likely the reason for these rocks to move, but they were rejected. Instead, some researchers claimed dust devils, thick sheets of ice, hurricane-force wind, or slick algal films were the likely contributors to the sailing stones. But unfortunately, these theories weren't backed by science.  

A combination of wind, temperature, and water: When paleobiologist  Richard Norris and his cousin Jim Norris traveled to Death Valley in December 2013, they found the playa covered by a seven cm (three inches) deep pond. Soon after, they witnessed the unexpected: The rocks started to move. Their findings demonstrate that moving the rocks involves a unique combination of circumstances. First, the playa must fill with water deep enough to form floating ice (chilly winter nights) but shallow enough to expose the rocks. Second, the pond freezes as nighttime temperatures drop, forming thin sheets of "windowpane" ice. Third, these ice sheets must be thin enough to move freely but thick enough to keep their strength. Lastly, on sunny days, the ice starts to melt and disintegrate into floating panels driven across the playa by gentle winds, pushing boulders in front of them around two to six meters per minute and creating trails in the soft subsurface mud. These findings nullified previous theories by solving the decades-old mystery. The Norris duo published their findings in a paper in the prestigious PLOS One journal in 2014.  

Death Valley's sailing stones continue to baffle scientists and visitors alike. Although the mystery seems to be solved, Death Valley may hold even more secrets for us to discover until proven!