The unexpected discovery of the cosmic microwave background by Arno Penzias and Robert Wilson in 1965 is now the centerpiece of our understanding of the Big Bang and the subsequent evolution of the universe. (SeePHYSICS TODAY, November 1997, page 32.) The discovery also set off something of a race to verify one of its implications for cosmic rays. In 1966, Kenneth Greisen (Cornell University) pointed out that the most energetic cosmic‐ray particles would be affected by interaction with the ubiquitous photons of this microwave background. Greisen predicted that, if cosmic‐ray sources were far enough away from us and if their energy spectrum extended beyond1020 eV,then the ultra‐high‐energy protons and nuclei would interact inelastically with the backgound radiation.