For centuries, visitors to Death Valley have marveled at the mysterious "sailing stones" of the Racetrack playa. Age-old rocks, some that weigh two or three times as much as a man, seemingly slid across the cracked mud of the dry lake bed, leaving distinct trails behind.

But did these stones actually move? Nobody has witnessed a rock in the playa stepping out for a slide. Scientists and the generally curious have been taking notes on the scene of these stones for decades.

About a year and a half ago, researchers from the Slithering Stones Research Initiative — an outfit started in 2011 by engineer Jim Norris and his cousin Richard, a paleobiologist at the Scripps Institute of Oceanography — set up shop at the Racetrack, a remote playa (dry lake bed) about 200 miles northwest of Las Vegas, across the California border. They placed video cameras in strategic locations. They measured. They put GPS monitors on some rocks.

And then they waited. As one member of the team told Jim Norris, it was "probably the most boring experiment ever."

But on an icy December day in 2013, the team just happened to be at the right place at the right time, with cameras rolling.

An aerial view of sailing rocks' trails on the Racetrack playaAn aerial view of sailing rocks' trails on the Racetrack playa. (Photo: Richard D. Norris, James M. Norris, Ralph D. Lorenz, Jib Ray, Brian Jackson/Wikimedia Commons)

The mystery behind the trails

Since the early 1900s, observers have offered different theories about the sliding rocks. Though many non-scientists have suggested something ghostly or paranormal, more grounded observers have considered strong winds, or ice, or rains that made the clay of the playa slippery, or some combination of those factors.

These guesses were alternately supported and slapped down. Wind, it was argued, would have to be extremely strong — maybe 100 mph — to move some of the rocks. The stones could travel, perhaps, on a thin level of ice, with enough rain, and enough wind, and at the right temperatures. But nobody had ever seen it.

In the early 2000s, a computer scientist and self-professed "skeptoid," Brian Dunning, caught video (below) of strong winds at the Racetrack after a rain, pushing water across the floor of the wide-open playa at surprising speeds. It was suggested then that maybe something stronger than water — say, ice — pushed at that speed might play a part on setting the stones sailing across the playa.

And then came the real scientists

Late in 2013, the Lewis cousins and their crew set up cameras and GPS-tagged some rocks. They hung around and hung around and hung around some more. And then, in December, the mystery was solved for all to see.

There had been some rain and snow in November — unusual in itself because the place gets only about 2 inches of rain per year — enough for a shallow pond to form on one end of the playa. The temperatures at night got cold enough so that the water was topped with a layer of ice in the morning.

The sun started to melt the ice by midday, and when a light breeze came down from the surrounding mountains and across the playa — a light breeze, not a 100 mph gale — the ice began to crack into huge sheets and float on the surface of the water.

When the massive sheets of ice, hundreds of feet across, bumped up against stones sitting on a thin layer of water on the clay floor of the playa — well, the Norris cousins and others had the cameras rolling and explained their findings in a paper in the online journal PLOS One, in August 2014.

"It's basically being like a tug boat or a bulldozer," Richard Norris told NPR last year. "It's pushing the rocks very slowly along."

The researchers noted many surprises in solving the mystery. A big one: The sheets of ice that pushed the rocks along were not bulldozer or iceberg-like. They were massive in terms of total size, but they were only 3-6 mm thick. Still, that was plenty to get the rocks moving.

The sleuths noted that the rock-movement was agonizingly slow at times, so slow that it was often missed by casual observation alone.

The sheer luck of catching it on camera was not lost on the researchers, either. Cold and wind are regular occurrences at the Racetrack for a few weeks every winter, they said in the paper, but getting it when enough standing water was present to form ice was almost too much to hope for. The stone movement didn't happen a few times a year, or every year, they posited. It might, they thought, be a decade or more between movements.

But they got it that winter. They recorded the proof, for everyone to see. And so the "sailing stones" of Death Valley's Racetrack playa — a wonder of nature, still — are no longer a mystery.