  This paper reviews some of the developments that over the last 10 years have
allowed us to go from deciphering the physical origin of several of the
enigmatic features of the second solar spectrum to discovering unknown aspects
of the Sun's hidden magnetism via sophisticated radiative transfer modeling.
The second solar spectrum is the observational signature of radiatively induced
quantum coherences in the atoms and molecules of the solar atmosphere. Magnetic
fields produce partial decoherence via the Hanle effect, giving rise to
fascinating observable effects in the emergent spectral line polarization.
Interestingly, these effects allow us to "see" magnetic fields to which the
Zeeman effect is blind within the limitations of the available instrumentation.
In the coming years, the physical interpretation of observations of the
spectral line polarization resulting from the joint action of the Hanle and
Zeeman effects might lead to a new revolution in our empirical understanding of
solar magnetic fields.
