  The hybrid design of the Pierre Auger Observatory allows for the measurement
of a number of properties of extensive air showers initiated by ultra-high
energy cosmic rays. By comparing these measurements to predictions from air
shower simulations, it is possible to both infer the cosmic ray's mass
composition and test hadronic interactions beyond the energies reached by
accelerators. In this paper, we will present a compilation of results of air
shower measurements by Pierre Auger Observatory which are sensitive to the
properties of hadronic interactions and can be used to constrain the hadronic
interaction models. The inconsistencies found between the interpretation of
different observables with regard to primary composition and between their
measurements and simulations show that none of the currently used hadronic
interaction models can provide a proper description of air showers and, in
particular, of the muon production.
