The lead coffin archaeologists found in the abandoned ancient city of Gabii, Italy could contain a gladiator or bishop. ANN ARBOR, Mich.—In the ruins of a city that was once Rome's neighbor, archaeologists last summer found a 1,000-pound lead coffin. Who or what is inside is still a mystery, said Nicola Terrenato, the University of Michigan professor of classical studies who leads the project—the largest American dig in Italy in the past 50 years. The sarcophagus will soon be transported to the American Academy in Rome, where engineers will use heating techniques and tiny cameras in an effort to gain...