Buried a mile into the South Pole ice, more than 5,000 sensors spread over a square kilometer lie in wait. Part of the IceCube experiment, they are a telescope of sorts that looks not for light, but for neutrinos coming in from the far-off cosmos.
Neutrinos are fundamental particles with no electric charge and almost negligible mass. Because of these properties, they are immune to the electromagnetic force and feel gravity only very minutely, and so they rarely interact with other matter. That property allows them to fly through space—even through Earth and our bodies—without impediment. But very rarely, one will hit an atom of polar ice head-on and release a spray of other particles, which in turn emit photons that light up IceCube’s sensors. If one struck an atom in your body (which should happen at least once in a lifetime to about a quarter of all people, one physicist estimates), you wouldn’t notice, but a similar reaction would occur.