For the fishes swimming deep in a particular patch of the Mediterranean Sea off the coast of Sicily in the wee hours of February 13, 2023, it was a night like any other—at least until a sudden azure shimmer, invisible to human eyes, shot through the dark water. The event signaled something extraordinary: the detection of the most energetic particle of its kind that has been measured to date.

The flash was the calling card of a cosmic neutrino, a tiny and typically mild-mannered “ghost particle,” so named because of its astronomical unlikelihood of interacting with the ordinary matter that makes up our world. A neutrino might pass through a light-year of lead unscathed. And each second some 100 trillion of these particles (most of which have been emitted by our sun) pass through your body.