On a hot afternoon in July 2012, Giuseppe Vercellotti was digging up bones near the wall of an abandoned medieval church here, thinking about getting a cold drink, when he heard his students call his name. Faces glistening with sweat, they told him that they had found something strange buried half a meter down. Vercellotti took a look and saw a layer of lime, used in ancient times to squelch the stench of rotting corpses. When he tapped the hard layer with his trowel, it sounded hollow.

“We immediately thought it was a mass grave,” says Vercellotti, a biological anthropologist at Ohio State University, Columbus, who co-leads a field school here. “We instructors were all excited and hopeful.”

But the students were apprehensive: “They all started talking about possible contagion,” Vercellotti says. Unconcerned, he leaned deep into the trench, where he got a whiff of a pungent odor and spotted an elbow bone poking out of the lime that had sealed it like a cast. The layer spoke of bodies tossed into a pit and hastily covered with lime. Could this trench hold victims of the Black Death, the plague that killed half of Europe in the Middle Ages?

It was the end of the summer field season. So the team carefully covered the trench with tarps and went home, hoping that excavations in 2013 would show that they had struck gold. They had come to expect extraordinary finds in the graveyard of the now decrepit Abbey of St. Peter, where a bountiful store of ancient skeletons was laid to rest in a single place over 1000 years, from the 11th to the 19th centuries.