During the first half of the first millennium A.D., the metropolis of Teotihuacan flourished in what is now central Mexico. Amid the bustling city’s formal avenues and standardized apartment blocks, stone faces measuring between six and nine inches wide were likely a common sight. Hundreds of such stone faces, once thought to have been masks buried with the dead, are now held by museums across the world. Most were fashioned from limestone, serpentinite, travertine, and liswanite, a rare form of volcanic rock. Recently, Smithsonian Institution scientists Timothy Rose and Jane MacLaren Walsh analyzed more than 100 of these stone faces. They concluded the faces were made outside Teotihuacan in workshops near stone sources. They also determined that the objects were not used as masks, but were likely central elements in larger ceremonial displays made of wood or other perishable materials that have not survived. Below are four of the stone faces examined by Rose and Walsh. (All photographs courtesy Jane MacLaren Walsh)