In Naples Cathedral on August 17, 1389, a priest lifted a sealed glass ampoule containing dark crystallised matter that was said to be the blood of San Gennaro, the bishop the Romans had beheaded under Diocletian a thousand years before. Famine had emptied the city. The procession was a plea for relief.

Witnesses said the matter inside the glass began to liquefy. It started to bubble. Some of it flowed.

On the strength of that one moment, the same ampoule has been brought out three times every year for the next six hundred and thirty six years. The expected schedule is the first Saturday of May, the 19th of September, and the 16th of December. Each time, in front of a crowd, the cardinal lifts the silver case and the brown solid inside the glass turns red and runs.

It works on television. It works in front of foreign delegations. It works while archbishops who do not believe in it watch.

When it fails, things tend to follow.

In September 1939, the blood did not liquefy. Italy entered the Second World War the following spring. In September 1943, it did not liquefy. The Nazis occupied Naples that month. In September 1980, it did not liquefy. Two months later, an earthquake centred 50 kilometres from Naples killed 2,483 people and left a quarter of a million homeless.

The Catholic Church has never officially declared any of this a miracle. The Church has also never permitted the ampoule to be opened.

Spectroscopic analysis in 1902 and 1989 detected the absorption bands of hemoglobin through the glass. Witnesses describe the brown solid frothing, bubbling, and rising in volume, behaviour simple gels do not show. Measurements in 1900 and 1904 recorded weight gains of up to 28 grams. Later precision balances detected none. Other Naples relics, the blood of Saints Patricia, Pantaleon, and Stephen, liquefy on their own feast days too.

In 1991 Italian chemists at CICAP reproduced a brown thixotropic solid using only materials a fourteenth century apothecary could obtain: iron chloride from Vesuvius, ground eggshell, salt and water. It was published in the journal Nature. It does not explain hemoglobin, foam, or weight. In their own laboratory the gel stops working after two years. In Naples it has been working for six hundred and thirty six.

It happens again in May, September, and December. The ampoule has not been opened in 636 years. The brown solid is still inside.

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