On the twelfth of December, 1531, a Mexican peasant unfolded his cloak in front of the bishop of Mexico City. An image of Mary was on it. The cloak is made of fibre that should rot within twenty years. It is now four hundred and ninety-five years old. The image is still there.

His name was Juan Diego. A widowed Aztec convert in his fifties. On the morning of the ninth of December, 1531, he was crossing Tepeyac Hill north of Mexico City when, he reported, he saw a young woman dressed in the sun. She asked him to take a message to the bishop. The bishop did not believe him. Three days later, on the twelfth of December, Diego returned to the hill. He found Castilian roses blooming in the December frost. He filled his cloak with them. He carried them back into the bishop's chamber. He unfolded the cloak.

The cloak is called the tilma. It is woven from the fibre of the agave cactus. Agave fibre in ordinary conditions lasts ten to twenty years before disintegrating. The tilma is now four hundred and ninety-five years old. The image on it has been studied repeatedly. In 1936, the German chemist Richard Kuhn, who would win the Nobel Prize in Chemistry two years later, analysed two fibres of the pigment. He reported that the pigment was not from any known natural source. Not mineral. Not animal. Not vegetable. In 1979, an American team led by the biophysicist Philip Callahan photographed the tilma in infrared light. They reported no preliminary sketch, no underdrawing, and no brushstrokes detectable anywhere on the cloth. Ophthalmologists who examined high-resolution photographs of the figure's eyes reported that they could see, reflected in the pupils, the small figures of Juan Diego, the bishop, and other men in sixteenth-century Spanish clothing.

The Catholic Church calls it a miracle. Science has not offered an agreed explanation for the cloth's preservation, the pigment, or the figures in the eyes. The tilma is on display today, behind bulletproof glass, above the high altar of the Basilica of Our Lady of Guadalupe in Mexico City. Twenty million pilgrims visit it every year. It is the most-visited Catholic shrine in the world. You can go and see it.

Witnessed. Subscribe.