The Renaissance poet Giovanni Boccaccio, best known for writing "The Decameron," also wrote a book on "100 Famous Women." No. 51 is Pope Joan.
Rare book dealers in Rome pull ancient tarot cards from their shelves. The card for hidden knowledge is "La Papessa" -- the Female Pope.
Travel north to Siena to the Duomo, where inside the cathedral is a gallery of terra-cotta busts depicting 170 popes, in no particular order. In the 17th century, Cardinal Baronuis, the Vatican librarian, wrote that one of the faces was a female -- Joan the Female Pope.
Baronius also wrote that the pope at the time decreed that the statue be destroyed, but some say the local archbishop didn't want a good to statue go to waste.
"The statue was transformed," believes Cross. "I mean, literally, it was scraped off, her name and written on top of Pope Zachary."
At the Basilica in St. Peter's Square are carvings by Bernini, one of the most famous artists of the 17th century. Among the carvings are eight images of a woman wearing a papal crown, and the images seem to tell the story of a woman giving birth and a baby being born.
Medieval manuscripts tell a similar tale: Two-and-a-half years into her reign, Pope Joan was in the midst of a papal procession, a three-mile trip to the Church of the Lateran in Rome, when suddenly at a crossroads, she felt sharp pains in her stomach.
She was having contractions, the stories say. The unthinkable happened -- the pope was having a baby.
"And then, shock and horror," says Malone. "And then the story gets very confused, because some of the records say she was killed and her child was killed right on the spot. Other records say she was sent to a convent and that her son grew up and later became bishop of Ostia."
Stories vary -- some say the crowd stoned her to death, others say she was dragged from the tail of a horse -- but in most accounts, Pope Joan perished that day.
In the decades that followed, the intersection was called the Vicus Papissa -- the Street of the Female Pope -- and for more than 100 years, popes would take a detour to avoid the shameful intersection.
Polonus writes: "The Lord Pope always turns aside from the street ... because of the abhorrence of the event."
The modern Catholic Church and many scholars dismiss the story of Pope Joan as a sort of Dark Ages urban legend.
Valerie Hotchkiss, a professor of medieval studies at Southern Methodist University in Texas, says that the story of Pope Joan was actually added to Martin Polonus' manuscript after he died.
"So he didn't write it, but it was put in very soon after his death, like around 1280 to 1290," says Hotchkiss. "And everyone picks it up from Martin Polonus."
Medieval monks were like copy machines, say some scholars, simply replicating mistakes into the historical record.
"And they're picking it up from each other and changing it and embellishing it," Hotchkiss says.
Monsignor Charles Burns, the former head of the Vatican secret archives, says the story intrigued people in the Middle Ages just as it intrigues people today. "This was almost like an Agatha Christie," he says, referring to the classic mystery writer.
Burns says there is no evidence and no documentation in the secret archives that Pope Joan existed, no relic of Pope John Anglicus anywhere.