On a flat dry lakebed in Death Valley National Park, heavy rocks sit at the end of long grooves they have plowed across the mud. The trails run for tens of meters, some bending in sharp turns or doubling back, yet no one had ever watched a rock actually move. For more than sixty years the question of how they travel sat unanswered, the subject of guesses that ranged from hurricane-strength winds to floating sheets of ice.

In 2014 a research team published the first direct scientific observation of the rocks in motion, and the mechanism turned out to be far gentler than the leading theories. The stones glide when a thin sheet of ice, only three to six millimeters thick, covers a shallow winter pond, starts to melt in the late morning sun, and breaks into floating panels that a light wind nudges across the water. The ice shoves the rocks along at a walking pace of a few meters per minute...