  The neutrino is the most elusive particle that we know and for many years
physicists doubted that neutrinos might never be revealed. Today we know and we
reveal neutrinos produced by different astrophysical objects and by
interactions of cosmic rays (natural neutrinos) or produced by nuclear reactors
and as secondary particles in accelerators (artificial neutrinos). This paper
focus on naturally occurring neutrinos, the disclosure of which requires
enormous experimental apparatus in underground laboratories, under water or
under the ice of the South Pole. They have allowed huge advances in
understanding of neutrino properties with the discovery of the oscillation
mechanism. And at the same time they opened new frontiers for the study of
astrophysics of the processes that produce energy inside the Sun; on the
mechanisms leading to stellar gravitational collapses; on astrophysical objects
that produce cosmic rays up to extreme energies.
