Muons are elementary particles that are formed high in the atmosphere by the interactions of cosmic rays with atomic nuclei up there. Muons are radioactive and have average lifetimes of about two-millionths of a second. Even though they travel at almost the speed of light, very few should be detected at sea level after traveling through the atmosphere-at least according to classical physics. Laboratory measurements, however, show that muons in great number do reach Earth's surface. What is the explanation?