Over 110 million years ago, during the Cretaceous period, the southern coastal area of South Korea, near the city Jinju, was covered by extensive lakes. The muddy shores were inhabited by frogs, lizards, turtles, and dinosaurs, all of which left their tracks in the muck. Whenever the water level rose, some of these footprints were filled with sand, allowing a fraction of them to be preserved.

Today, thousands of tracks can be found in this area, known as the Jinju Formation, says Martin Lockley, a paleontologist who specializes in trace fossils like footprints—known as an ichnologist—at the University of Colorado Denver. Lockley and colleagues in South Korea have studied the tracks at Jinju for decades, and for many years, they’ve been