Earth is continually pelted by high-energy particles coming mostly from beyond the solar system. These cosmic rays, as they are known, typically travel at speeds 90 to 99 percent that of light. Protons constitute the vast majority of cosmic rays, although the nuclei of helium and heavier elements make up roughly 10 percent and a few percent come as electrons and positrons. Astronomers suspect that most of these particles form in supernova explosions, which blast them out into the galaxy.
When cosmic rays reach Earth, they smash into the nuclei of atoms in the atmosphere, giving rise to a host of secondary particles. These eventually reach Earth's surface, where they can overwhelm detectors designed to pick up the subtle signatures of neutrinos and other subatomic particles suspected of contributing to dark matter. That's why physicists usually shield such detectors by burying them deep underground.