  The origin of ultra high energy cosmic rays promises to lead us to a deeper
understanding of the structure of matter. This is possible through the study of
particle collisions at center-of-mass energies in interactions far larger than
anything possible with the Large Hadron Collider, albeit at the substantial
cost of no control over the sources and interaction sites. For the extreme
energies we have to identify and understand the sources first, before trying to
use them as physics laboratories. Here we describe the current stage of this
exploration. The most promising contenders as sources are radio galaxies and
gamma ray bursts. The sky distribution of observed events yields a hint
favoring radio galaxies. Key in this quest are the intergalactic and galactic
magnetic fields, whose strength and structure are not yet fully understood.
Current data and statistics do not yet allow a final judgment. We outline how
we may progress in the near future.
