One of the most important places of worship in the city, the basilica of San Francesco has kept and has seen perpetuating for almost three hundred years what the Catholic Church defines as a permanent Eucharistic miracle, that is, a prodigious event still in progress involving the Eucharist.
On the evening of 14 August 1730 the friars minor convent of San Francesco placed a silver pyx with 351 consecrated particles inside the tabernacle of one of the chapels of the basilica – the one dedicated to Saint Anthony – and then, as every year, they closed the door of the church and went to the cathedral, together with the entire Sienese population, for the celebration of vespers on the eve of the feast of the Madonna Assunta. The next day, during the Eucharistic celebration, the terrible crime was discovered: the pyx with the particles had been stolen. The news of the sacrilegious gesture spread throughout the city and that day, which has always been characterized by great joy for the feast of the Assumption, was marked by such profound dismay that it was even decided not to run the prize on August 16.
A research conducted by the civil and religious authorities immediately began until, on 17 August, in the nearby collegiate church of Santa Maria in Provenzano, during mass a cleric accidentally caught a glimpse of something white inside the alms box; after referring it to the sacristan and archbishop Alessandro Zondadari and the vicar general rushed, the box was opened and the particles were found inside. Once examined and identified as the stolen ones, the city finally felt freed from a burden and all the people rushed to pay homage to them. A solemn procession was thus organized and in a riot of songs and prayers, accompanied by an immense crowd, the hosts were brought back to San Francesco, where the question of what to do with them arose. The friars did not want to destroy them because they had been consecrated and therefore contained the presence of Christ, but they could not even use them because they had been inside a dirty box for days, among money and dust. It was therefore decided to let nature take its course, waiting for them to deteriorate on their own. Almost three centuries have passed since then and those particles, against all chemical and biological laws, are still miraculously preserved intact.