Cosmic Rays
Cosmic rays are fast-moving particles from space that constantly bombard the earth from all directions. Most of the particles are single protons and electrons but few are much heavier nuclei, ranging up to lead. Cosmic ray particles travel at nearly the speed of light, and some are the most energetic particles ever observed in nature. The highest-energy cosmic rays have a hundred million times more energy than the particles produced in the world's most powerful particle accelerator.