I'm standing on a catwalk in a giant cave crammed with industrial equipment, and I'm told that trillions of neutrinos are flying through every inch of my body each second. I reach out my arms as if to heighten the sensation, but of course, I can't feel a thing. Nearly massless, traveling close to the speed of light, the ghostly particles traverse the empty space between my atoms without a trace. They also move mostly unimpeded through the hulking metal box that dominates the cavern. But a few times a day one will collide with an atom inside the school bus–size contraption, liberating charged particles that leave light trails visible to scientists. And these trails, physicists hope, will lead them into unknown territory.