Hidden inside the narrow growth rings of Pyrenees trees lies the strongest evidence yet for what set the Black Death in motion. For the first time, researchers have combined high-resolution climate reconstructions with medieval records to draw a direct connection between a sudden climatic shock and the arrival of the plague in Europe, where it killed tens of millions between 1347 and 1353.

A volcanic eruption may have set off a chain reaction that led to Europe's deadliest pandemic.