For 100 years, 700-pound rocks seemed to move on their own in California's Death Valley. In 2013, scientists finally saw the strange force that was pushing them

For decades, visitors to California's Death Valley came across a strange sight at Racetrack Playa. Huge rocks, some weighing as much as 700 pounds, appeared to have moved across the flat, dry lakebed on their own, leaving long trails behind them. There was no obvious explanation. No one had actually seen the rocks moving, and some of the tracks stretched for hundreds of metres.

It became one of Death Valley's most unusual mysteries, with theories ranging from powerful winds and dust devils to ice and even less scientific explanations. Then, in 2013, researchers finally happened to be in the right place at the right time.

The rocks that appeared to move by themselves

Racetrack Playa is a dry lakebed in Death Valley National Park. Rocks that have fallen from the nearby mountains sit across its flat surface, and some of them have left trails as long as 1,500 feet.


The rocks are often called "sailing stones". Some are relatively small, while others are enormous. The National Park Service says some can weigh up to 320 kilograms, or about 700 pounds.

The first serious scientific investigations went back decades, but the biggest problem was simply catching the movement. The rocks could remain in one place for years before conditions were right.

Scientists had considered wind as one possible answer. But moving the biggest rocks directly with wind would require extremely strong gusts, making that explanation difficult to accept on its own.

A simple experiment gave scientists a clue

Planetary scientist Ralph Lorenz became interested in the rocks while working with weather stations in Death Valley. His interest eventually led to a surprisingly simple experiment.

He froze a small rock into a block of ice and floated it in a tray containing water and sand. When he gently blew across the ice, the rock moved across the sand and left a trail.

The experiment suggested that the ice did not need to lift the entire rock off the ground. Instead, ice could reduce the resistance enough for a relatively light wind to push the rock along.

As Smithsonian reported, Lorenz developed the idea that a thin floating sheet of ice could move around a rock and help it travel across the wet surface.

But there was still one major problem. Nobody had actually watched the real rocks do it.

Then the rocks finally moved

In 2011, paleobiologist Richard Norris and his team began monitoring Racetrack Playa. They installed a weather station and placed GPS equipment on 15 rocks brought in for the experiment.

The researchers thought they might have to wait a decade or longer. Instead, in December 2013, the conditions suddenly became perfect.

A winter storm had left a shallow pond on the playa. During the cold nights, the water froze into extremely thin sheets of ice. When the sun came out, the ice began breaking apart. Then the wind took over.

The researchers watched floating pieces of ice being pushed across the water by relatively light winds. Those ice panels, some measuring tens of metres across, pushed rocks resting on the muddy lakebed. The stones moved slowly, leaving fresh trails behind them.

"Science sometimes has an element of luck," Richard Norris said. "We expected to wait five or ten years without anything moving, but only two years into the project, we just happened to be there at the right time to see it happen in person."

The strange part was how little force was needed

The researchers found that the rocks did not require hurricane-force winds. They moved when thin sheets of ice, only about 3 to 6 millimetres thick, were pushed by winds of roughly 4 to 5 metres per second.

The rocks themselves moved at only a few metres per minute, which is slow enough that a person standing nearby might not realise they were watching a rock move at all. Some rocks travelled more than 60 metres during individual events, while others moved several times before finally stopping.

The mystery, therefore, was not that the rocks had somehow learned to move by themselves. The real explanation was much stranger in its own way: a rare meeting of shallow water, freezing temperatures, thin floating ice and a light desert wind.