A pot of gold at the end of the cosmic "raynbow"?
  We critically review the common belief that ultrahigh energy cosmic rays are protons or atomic nuclei with masses not exceeding that of iron. We find that heavier nuclei are indeed possible, and discuss possible sources and acceleration mechanisms for such primaries. We also show detailed simulations of extensive air showers produced by ``superheavy'' nuclei, and discuss prospects for their detection in future experiments. 
L. A. Anchordoqui, M. T. Dova, T. P. McCauley, T. Paul, S. Reucroft,   and J. D. Swain