GPR surveys like his have already upended long-held assumptions about the city, including the history of one of its most visited landmarks. Located on the Via Stabiana on the southern edge of town, the Quadriporticus is an enormous open-air atrium, about an acre in total, surrounded by a colonnade and dozens of small rooms. For years, investigators assumed, based on a single fresco of weapons, that it had been a gladiator barracks. Ellis and Poehler’s GPR analysis, however, offered a more compelling clue: About 1.5 feet below the surface sits a circular structure that was once the foundation for a large, central gazebo. The form matches ones found locally and others at outdoor markets in Rome. This suggests that the ancient square might have been filled with vendors selling fermented fish, sauces, and wine. Maybe it wasn’t a place for entertaining the rich but for feeding the masses.