Number 84 (Story #3), June 15, 1992 by Phillip F. Schewe and Ben Stein GROUND-BASED GAMMA-RAY ASTROPHYSICS looks at physics in energy doses up to the PeV (10**15 eV) and even the EeV (10**18 eV) range. The sources for such stupendous energies are believed to be compact objects such as black holes or neutron stars. The messengers are high-energy gamma rays which, having no electrical charge, can follow a straight path through the galactic magnetic field. The evidence for the gammas here on Earth are the huge showers of particles engendered by the gammas when they hit our atmosphere. To search for PeV gammas, a Michigan-Chicago-Utah collaboration uses a combination of 1089 scintillation detectors, muon detectors, and Cherenkov telescopes. The University of Utah's Fly's Eye Detector looks for the fluorescence of air molecules arising from very-high-energy gammas; they observe about 20 events per year with energies above 10 EeV. (Beam Line, Spring 1992; published by SLAC.)