A water wave is like a traveler. Sometimes it gets tired and spreads out along the way, sometimes it moves steadily, and sometimes it balls up and runs faster. Scientists tested this in a large tank 50 meters long. They created a wave shaped like a hill and varied only its height, keeping the width the same.

When the hill was low, it spread out: different parts of the wave traveled at different speeds, and it lost its shape. But when the height was chosen precisely, the wave traveled unchanged, as if it were that very stable solitary wave. And when the hill was made even taller, the wave began to compress: its crest grew, and its base narrowed. The peak overtook the rest of the group, like a runner who breaks ahead.

Previously, it was unclear why the same wave behaves differently. Now it is clear: it is all about the balance between height and width. If they match each other, the wave travels steadily. This knowledge helps predict dangerous waves in the ocean and tune light pulses in optical fibers. The traveler, it turns out, can not only get tired but also hold its shape, and sometimes ball up.