Fifteen hundred years ago, Mexico’s Teotihuacan was a multicultural metropolis, enlivened by the diverse dress, foods, and dialects of its immigrant groups. Artifacts show the city of more than 100,000 depended on a steady stream of foreigners, who brought skilled labor and exotic goods from across Mesoamerica. But after Teotihuacan faded, during a period of upheaval and uncertainty, locals may have turned against outsiders—and archaeologists now think they’ve found the decapitated heads to prove it.