Januarius of Benevento
Bishop and Martyr · 272–305
“The blood of the martyrs is the seed of the Church.”
Tertullian, Apologeticus, Chapter 50 (attributed in the tradition surrounding the Neapolitan martyrs)
Januarius was the bishop of Benevento during one of the worst stretches of Roman persecution: the reign of Diocletian. When he heard that two of his deacons had been arrested, he didn't sit tight and hope for the best. He went to visit them in prison. That visit got him arrested too.
Around 305, he was brought before the governor Timotheus in Pozzuoli. The stories say he survived being thrown to wild beasts in the amphitheater (the animals reportedly wouldn't touch him), and was eventually beheaded along with several companions near the Solfatara volcano outside Naples.
Here's where it gets interesting. A woman named Eusebia is said to have collected some of his blood in two small glass vials. Those vials ended up in Naples, and they've been there ever since. Several times a year, including on his feast day, the cathedral brings out the vials. And the dried, solid blood liquefies. It's been happening since at least 1389, in front of crowds, under scientific scrutiny, with no explanation that holds up. The Church doesn't require you to believe it's miraculous, but she hasn't explained it away either.
Naples takes this seriously. When the liquefaction doesn't happen, people get nervous. There's a long tradition of linking failed liquefactions to disasters, wars, and outbreaks. Whether that's superstition or pattern recognition is a question worth sitting with.
Januarius is the kind of saint who makes you realize that faith has always lived right next to mystery.