  The Pierre Auger Observatory is the largest extensive air shower detector.
Based on a hybrid system, this experiment measures the longitudinal shower
development and the particles at the ground. This detection system allows the
extraction of the p-air cross-section at energies much higher than the ones
accessible by current colliders. It is also possible to test hadronic
interaction models using correlations between different air shower observables,
like the depth of shower maximum and the muon number at the ground and their
fluctuations. Thanks to the low energy extension of the Pierre Auger
Observatory, the muon deficit in air shower simulations can be addressed over
almost three decades at the highest energies.
