“Soft rock and maybe brick. I’m hoping we don’t need it. Power tools were actually banned by the pope when he authorized the modern excavation, to avoid any collateral damage, but we’re not going to worry about that detail. Where we’ll be working, the only thing we might damage is the ossuary itself.”

“Aren’t you expecting we’ll be digging in just plain dirt?” Sana asked. In her mind, the idea of cutting into rock made the scope of the project significantly more daunting.

“No, it’s going to be more like hardpan, a claylike layer mixed with gravel but highly compacted to seem like very soft stone. As I mentioned, the tomb that Peter’s followers made for him on the Vaticanus hill adjacent to Nero’s circus was an underground chamber with a barrel vault. They dug a large hole and then built two parallel brick foundation walls oriented in an east-west direction. Saturninus’s letter says that the ossuary was placed midway at the base of the north wall and concealed before the excavation hole outside the walls was filled back in.”

“And the base of the north wall is where we’re going to find the ossuary?”

“That’s right. During the last major excavation, more than fifty years ago, the archaeologists tunneled under that north wall to get inside the original tomb’s chamber, to avoid destroying the mishmash of graves, altars, and trophies clustered above Peter’s underground tomb. Starting from soon after his death until not that long ago, people clamored to be buried as close as possible to him. Anyway, it’s in the roof of that tunnel where we are going to find the ossuary.”

“I’m having trouble picturing all this.”

“For good reason. Soon after Peter’s death, the whole hill became not just the place for future popes to be buried but a popular Roman necropolis filled with graves and mausoleums. Today, because of its location beneath Saint Peter’s, only a small portion of it has been excavated. And within a twenty-foot or so cubic area right around Peter’s tomb, there is such a hodgepodge of ancient construction, you can’t believe it. To make things more complicated, sometime in the first century a monument called the Tropaion of Peter was built just above his grave. Then in the fourth century, Constantine built his basilica around this monument, using it as an altar. During the Renaissance, Saint Peter’s was built on top of Constantine’s basilica, locating the high altar directly atop what had been Constantine’s altar, now some forty feet above the floor of Peter’s original tomb.”

“It sounds like a layer cake,” Sana said.

“That’s a good analogy,” Shawn agreed.

Once inside the terminal and through passport control, Shawn and Sana split up, with Sana heading for the baggage area and Shawn for the rental-car stands. Within half an hour they were on their way.