  Extensive air showers are the result of billions of particle reactions
initiated by single cosmic rays at ultra-high energy. Their characteristics are
sensitive both to the mass of the primary cosmic ray and to the fine details of
hadronic interactions. Ultra-high energy cosmic rays can be used to
experimentally extend our knowledge on hadronic interactions in energy and
kinematic regions beyond those tested by human-made accelerators.
  We report on how the Pierre Auger Observatory is able to measure the
proton-air cross section for particle production at a center-of-mass energy per
nucleon of 39 TeV and 56 TeV and also to constrain the new hadronic interaction
models tuned after the results of the Large Hadron Collider, by measuring: the
average shape of the electromagnetic longitudinal profile of air showers, the
moments of the distribution of the depth at which they reach their maximum, and
the content and production depth of muons in air showers with a primary
center-of-mass energy per nucleon around and above the 100 TeV scale.
