  The hadronic final state of central Pb+Pb collisions at 20, 30, 40, 80, and
158 AGeV has been measured by the CERN NA49 collaboration. The mean transverse
mass of pions and kaons at midrapidity stays nearly constant in this energy
range, whereas at lower energies, at the AGS, a steep increase with beam energy
was measured. Compared to p+p collisions as well as to model calculations,
anomalies in the energy dependence of pion and kaon production at lower SPS
energies are observed. These findings can be explained, assuming that the
energy density reached in central A+A collisions at lower SPS energies is
sufficient to force the hot and dense nuclear matter into a deconfined phase.
