SAILING STONES
A big boulder sticks out of the dried mud. This rock is too heavy to pick up, yet it has mysteriously moved. Once, it was 250 m away. Now, a trail shows where it moved across the dirt. This isn’t the only rock on the move here. Many rocks in this part of Death Valley 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. No one has ever seen these rocks move. It’s a mystery.
Searching for a Cause Over the years, people have come up with wild ways to explain how the rocks move. Some 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. T is force could tug on the rocks and pull them downhill. So scientists looked for patterns in which
direction the rocks moved. T at’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. T e winds here are strong. Some gust up to 113 kph through Death Valley. Windstorms called dust devils also whirl. T e scientists did experiments to test the
wind theory. One even used the wind from a plane propeller to try to move the rocks. T e tests showed that winds aren’t strong enough to move the biggest boulders.
12 NATIONAL GEOGRAPHIC EXTREME EXPLORER ★
Cold Clue Finally, scientists noticed that the rocks seem to move only in the winter. T at’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. T e 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 T e water gave scientist Ralph Lorenz a diff erent idea. If it got cold enough in the winter, rings of ice could form around the rocks. Ice floats on water, so the ice might liſt even the biggest rocks a little. T en a light wind could push them. Lorenz made a model of his idea to test his
theory. 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. T e rock floated a little bit. T en 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.
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