  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.

Astrophysics;High Energy Physics - Phenomenology