The great majority of cosmic‐ray particles reaching Earth's surface are positive and negative muons. At sea level, the muon flux is about104/m2per minute. These muons are mostly decay products of short‐lived &pgr; mesons created by cosmic‐ray protons hitting the upper atmosphere. Because muons are 200 times heavier than electrons and impervious to the strong nuclear force, they are far more penetrating than electrons, neutrons, x rays, or even gamma rays. With a typical energy of 3 GeV, a cosmic‐ray muon will pass through 15 m of water or 2.5 m of steel.