About 4,000 years ago, people started burning the forest, which was mostly bamboo, just enough to make small openings. They may have planted maize or squash, weeded out some underbrush, and transported seeds or saplings to create a partly curated forest of useful tree products that Dr. Watling calls a “prehistoric supermarket.” After that, they started building the geoglyphs. The presence of just a few artifacts, and the layout of the earthworks, suggest they weren’t used as ancient villages or for military defenses. They were likely built for rituals, some archaeologists suspect.