More than 125 of these curious soccer ball-sized glass spheres hang near the floor of the Mediterranean Sea. Even though they're deep underwater, they're keeping a constant vigil for neutrinos, particles that may be evidence of dark matter, supernovae, and neutron stars far off in intergalactic space. Eventually, the Cubic Kilometer Neutrino Telescope (KM3NeT) will consist of 6,000 spheres suspended across one cubic kilometer of seawater. Often, neutrino detectors are located deep underground because the Earth itself helps isolate the instrument from background radiation and cosmic rays so the neutrinos are more easily spotted. From Scientific American:
"Perhaps one or two neutrinos in a million will interact with quarks inside the nucleus of either hydrogen or oxygen" in the water, says the project's physics and software manager, Paschal Coyle of the Marseille Particle Physics Center. "Because the cosmic neutrinos possess very high energy, the result of such interactions is the release of a charged particle that travels very fast."
In fact, it travels through the water faster than light can, producing an effect Coyle likens to an optical equivalent of the Concorde jet's sonic boom. Researchers can determine the original neutrinos' energy and direction using the faint light released—so-called Cherenkov radiation—picked up by the undersea sensors.