Nature produces cosmic rays up to macroscopic energies of at least 10^20 eV. While their origin at low to moderate energies appears reasonably well understood, those with extremely high energies remain mysterious. They are thought to be of extragalactic origin but their interaction with the cosmic microwave background ration (CMB) prevents them to travel over cosmological distances. As an alternative, it has been proposed that they may originate from decays of topological defects created in the early Universe.

The Pierre Auger Cosmic Ray Observatory,  nearing completion in Argentina, aims at solving this most pressing question astroparticle physics.  The concept and status of the experiment and first science data will be presented.