  The Pierre Auger Observatory is the world's largest detector of ultra--high
energy cosmic rays (UHECRs). It uses an array of fluorescence telescopes and
particle detectors at the ground to obtain detailed measurements of the energy
spectrum, mass composition and arrival directions of primary cosmic rays (above
the energy of $10^{17}$ eV) with accuracy not attainable until now.Observations
of extensive air showers performed by the Pierre Auger Observatory can also be
used to probe hadronic interactions at high energy, in a kinematic and energy
region not accessible by man-made accelerators. Indeed, exploiting Auger data,
we reach center-of-mass energies up to 400 TeV, i.e. more than 30 times of
those attainable at the LHC, and explore interactions in the very forward
region of phase space on targets of atomic mass of 14. In addition, a precise
measurement of the muon component of air showers at the ground is more
sensitive to the details of the hadronic interactions along many steps of the
cascade development, such as the multiplicity of the secondaries and the
fraction of electromagnetic component with respect to the total signal. On the
other hand, the intrinsic muon fluctuations mostly depend on the first
interaction. In this paper we overview the new Auger studies exploring the
connection between the dynamics of the air shower and the multi-particle
production, and how this knowledge can be translated into constraints of the
high energy hadronic models as well as direct measurements, complementary to,
and beyond the reach of, accelerator experiments.
