California’s Death Valley is famous for its extreme record-setting temperatures and otherworldly landscapes. At one in particular, the Racetrack Playa, there can be seen an especially strange phenomenon: large stones situated on the flat expanse have long tracks etched into the mud behind them, giving the appearance that the stones have moved across the playa on their own.

Weighing up to 700 pounds (318 kg), the stones are large, and they travel far, as many as 820 feet (250 meters). Adding to the strangeness, up until a few years ago, no one had ever actually seen them move – there was only the evidence of it in the trails they left behind.

In these trails are curious clues to their movements: some of the trails are smooth and slightly rounded, while others are straight with sudden shifts from side to side. Many of the tracks also run parallel to each other, even turning and changing directions with each other. Then, they can remain stationary without moving again for a decade or more.

For more than 70 years, scientists were baffled by this phenomenon and tried to find out what could possibly be causing it, giving rise to several potential explanations such as dust devils, hurricane-force winds, or even magnetic fields. Without any proven conclusion, among the public superstitious causes such as space aliens also began to emerge, and they persist to this day, even with the prevailing scientific explanation first discovered in 2006.

The process is essentially this: rocks fall from the surrounding mountains into the playa, which is actually a dried lake bed. However, in the winter, when conditions are right, water is present in the lake bed and overnight forms a thin, frozen layer of ice molded around the rocks. During the day, the ice melts just enough to break off into floating slabs – with the rocks still encased in them —  and just a slight breeze is enough to set these ice/rock formations into motion across the playa and leave trails in the mud behind them.

This explanation was originally not formed by real-life observations but by piecing together related observations and experimentation by Ralph Lorenz, a NASA scientist. Lorenz began his research by noticing a similar phenomenon among beaches along the Arctic Sea, where large rocks are buoyant and reach new destinations by floating on the water with ice. Inspired by this, he devised an experiment to show how the same mechanism could be in effect in Death Valley. The model was simple, and you could even try it yourself at home: Lorenz placed a rock inside a Tupperware container with an inch of water, then put the container in the freezer. Once frozen, the rock was encased in ice, and he removed this ice/rock slab and put it in a larger tray of sand and shallow water. Then, just by gently blowing on it, the rock/ice mixture moved across the tray.

Though this became the generally accepted explanation, it didn’t catch on for years due to the fact that still no one had witnessed the rocks actually moving in the playa. It wasn’t until 2014, when the stones’ movement was finally tracked and recorded, that this process was confirmed, this time by researcher Dr. Brian Jackson of Boise State University. Over the course of two and a half months, the research team used time-lapse photography to capture images of hundreds of stones and track their movements, which are slow – the stones move only 2-6 meters per minute, and remain in motion for only seconds or minutes.

You can watch their fascinating, hypnotic movements and see additional photos in the video below:

While science has satisfactorily explained the mechanics of this one, there still seems to be an element of mystery. For example, in one observation, stones that were 900 feet (300 meters) apart began moving at the same time, and they traveled over 180 feet (60 meters) before stopping. So even though we may know how they are moving, their apparently synchronous coordination suggests that we still don’t know exactly why.