On a dry lakebed in Death Valley National Park called Racetrack Playa, hundreds of rocks sit at the ends of long, shallow trails. Some trails run straight for hundreds of feet; others curve or zigzag. Some of the rocks weigh more than a hundred pounds. For decades, nobody had ever seen one move.

People offered explanations of every kind. Some suggested powerful winds; others proposed that sheets of ice had dragged the rocks, or that the ground itself shifted. A few blamed pranksters. Each idea had a weakness. Winds strong enough to push a heavy rock across dry ground would have to be far stronger than any ever measured there, and no one could explain how pranksters would leave no footprints in the soft mud.

In the early 2010s, a team of researchers decided to stop guessing and start watching. They placed rocks fitted with GPS trackers on the playa, set up a weather station and time-lapse cameras, and waited. Then, one winter, it happened. A shallow layer of rainwater had collected on the playa and frozen overnight into ice only a few millimeters thick. As the morning sun warmed it, the ice broke into large, thin panels. A light breeze pushed the floating panels, and the panels pushed the rocks, which slid slowly through the mud beneath the water and carved new trails.

The answer turned out to combine pieces of several older ideas: ice, wind and water all played a part. But none of those ideas had been right on its own, and it took patient observation, not clever arguments, to show how the pieces fit together.