Tens of thousands of people once lived in Cahokia, the city at the heart of the mound-building Mississippian culture (which dominated the midwestern and southeastern United States from 700 to 1500 CE). And then, around 1450, they all left. Now, sediment cores from nearby Horseshoe Lake suggest that the area didn’t stay deserted for long.

Wait, fecal chemicals last how long?

The study looked for the chemical signature of ancient human feces, which washed into nearby Horseshoe Lake over the centuries along with layers of soil, pollen, and other material. When bacteria in your gut break down cholesterol, they produce a chemical called coprostanol, which can survive in soil for hundreds or even thousands of years. More coprostanol in the soil means more people living (and pooping) in the area around the lake.

If you want to get really technical, archaeologist A.J. White of the University of California, Berkeley, and his colleagues actually measured the ratio of coprostanol to another chemical called cholestenol, which is formed when soil microbes break down cholesterol in the soil. A higher ratio means more human waste. That ratio can’t measure population sizes, but it can tell researchers whether populations were increasing or decreasing—and how quickly.