Soon after the space shuttle atlantis launched a new observatory into orbit in 1991, Gerald Fishman of the NASA Marshall Space Flight Center realized that something very strange was going on. The Compton Gamma Ray Observatory (CGRO), designed to detect gamma rays from distant astrophysical objects such as neutron stars and supernova remnants, had also begun recording bright, millisecond-long bursts of gamma rays coming not from outer space but from Earth below.