The Duomo di Napoli, properly the Cathedral of Santa Maria Assunta, guards a puzzle behind its altar: two phials of blood that liquefy, apparently on cue, three times a year.

The blood is said to belong to San Gennaro, Naples's patron saint, martyred in the early fourth century and invoked more readily than any deity. Legend credits him with keeping Vesuvius in check. First recorded in 1389, the ritual persists, its failure read as a bad omen. Only one person has ever opened a phial: an eighteenth-century king who pilfered a taste, leaving it half empty.

Behind this spectacle sits a building layered since the thirteen hundreds around the older basilica of Santa Restituta, whose baptistry is billed among the oldest in the Western world. VoiceMap's Blood and Passion in Naples tour uses the cathedral to trace the science, superstition and royal mischief behind the city's strangest ritual.