Sailing stones

A big boulder sticks out of the dried mud. This rock is too heavy to pick up. Yet, it has mysteriously moved. A few days ago, it was 250 meters (820 feet) away. A trail shows where it moved across the dirt.

This isn’t the only rock on the move here. Many of the rocks in this part of Death Valley in North America don’t stay in one place. Their trails crisscross the cracked mud of a dried lake. 

Some glide in straight lines, then stop and settle into the dry mud. Some rocks move in pairs. Others zigzag this way and that, leaving a jagged trail. How did they move? 

No one has ever seen it. It’s a mystery.

Boulders mysteriously zig and zag in Death Valley, California, U.S.A.



A man takes a photograph of boulders and their trails.

for a Cause

Over the years, people have come up with wild ways to explain how the rocks move. Some people say the rocks are magical and move on their own. Others say aliens from space are moving them. Still others think pranksters are playing tricks, moving the rocks when no one is looking.

Scientists want to find a better answer than magic or aliens or pranksters. So they’ve looked for evidence in nature for answers.

Looking to Nature

An early theory was that gravity moves the rocks. This force could tug on the rocks and pull them downhill. Even though this area looks flat, it actually slopes a little.

So scientists looked for patterns in which direction the rocks moved. That’s when the gravity theory fell apart. Most of the rocks had moved uphill. Gravity is a strong force, but it pulls things down, not up. Another theory was that winds pushed the rocks. The winds here are strong. Some gust up to 113 kilometers (70 miles) per hour through Death Valley. Windstorms called dust devils also whirl.

The scientists did experiments to test the wind theory. One even used the wind from a plane propeller to try to move the rocks. The tests showed that winds aren’t strong enough to move the biggest boulders.

This video follows the trail made by a heavy stone.

Cold Clue

Finally, scientists noticed that the rocks seem to move only in the winter. That’s when this area sometimes floods. A thin layer of water covers it and surrounds the base of the rocks.

Studying the water led scientists to new theories. Geologist Paula Messina observed a thin layer of slimy bacteria growing in the water. The slime makes the ground really slippery, so the rocks can slide easily. Even a small wind may be able to push the rocks.

An Experiment

The water gave scientist Ralph Lorenz a different idea. He thought that it might get cold enough in the winter for rings of ice to form around the rocks. Since ice floats on water, the ice might lift even the biggest rocks a little. Then a light wind could
push them.

Lorenz did an experiment to see if his theory worked. He made a model of his idea. First, he put a rock and some water in his freezer. As soon as a ring of ice froze around the rock, he placed it in a tray of water with sand on the bottom. The rock floated a little bit. Then Lorenz blew lightly on the rock. It moved! Its bottom scraped along the sand, leaving a trail.

So it seems that ice and wind could help the rocks move. So could bacteria and wind. Both theories are possible. Yet no one’s ever seen the rocks move, so this mystery remains unsolved.

Scientist Ralph Lorenz stands near a flooded area of Death Valley.