The cloak worn by St. Juan Diego Cuauhtlatoatzin was made from rough cactus materials from central Mexico and it should have deteriorated after 15-30 years.

But this "tilma" remains intact and its mysterious image of the Virgin Mary has not faded since December 1531, when the indigenous peasant reported a series of Marian encounters. The framed cloak is displayed behind the high altar of the Basilica of Our Lady of Guadalupe at the foot of Tepeyac Hill in Mexico City.

Scientists have studied the cloak for centuries. For starters, it's hard to describe the survival of this cactus-fiber cloak without using the word "miracle."

"We are dealing with mysterious events, but that doesn't mean they aren't real," said Vivian Dudro, a senior editor at Ignatius Press who helped produce a new edition of "The Lady of Guadalupe," a classic children's book by the late artist Tomie dePaola.

"All I know is that historians and scientists keep digging into the details of all this. Even with what we call 'legends,' you soon realize that there are real people involved in stories of this kind," she said, in a telephone interview. "The story of Our Lady of Guadalupe is best described as 'sacred history,' and pieces of this history continue to emerge to this day."

Year after year, Juan Diego's tilma is viewed by an estimated 20 million pilgrims, with more than 10 million visiting the basilica close to December 12 -- the feast of Our Lady of Guadalupe, the patron saint of Mexico and the Americas. Around the world, throngs march in parades and sacred processions behind copies of this iconic Marian image.

While Our Lady of Guadalupe has played a central role in Mexico's tempestuous history, Pope Francis has stressed that this image should not be tethered to culture and politics. "The message of Guadalupe does not tolerate any ideology of any kind," he said, during last year's Vatican rites for the feast day. Instead, believers should focus on Mary's question to Juan Diego: "Am I not here, I, who am your mother?"

This is a key message, said Dudro, that children need to hear when parents and teachers introduce them to the story of Juan Diego, the Castilian roses he plucked -- following Mary's instructions -- from the frozen soil and, finally, the image of her that appeared on his cloak when the roses spilled out before the Franciscan bishop of Mexico.