  It is pointed out that the hadron spectroscopy should be a study of the
structure of the QCD vacuum, low-energy elementary excitations on top of which
are hadrons. Concentrating on the dynamical breaking of the chiral symmetry in
the QCD vacuum, we emphasize the importance to clarify what is going on with
mesons in the $I=J=0$-channel, i.e., the sigma meson channel, because it is
connected to the quantum fluctuations of the chiral order parameter. After
summarizing the significance of the sigma meson in QCD and low-energy hadron
phenomenology, we give a review on some theoretical and experimental effort to
try to reveal the possible restoration of chiral symmetry in hot and dense
nuclear matter including heavy nuclei.
