  This paper reviews some of the most recent advances in the application of the
Hanle effect to solar physics, and how these developments are allowing us to
explore the magnetism of the photospheric regions that look ``empty'' in solar
magnetograms--that is, the Sun's ``hidden'' magnetism. In particular, we show
how a joint analysis of the Hanle effect in atomic and molecular lines
indicates that there is a vast amount of hidden magnetic energy and unsigned
magnetic flux localized in the (intergranular) downflowing regions of the quiet
solar photosphere, carried mainly by tangled fields at sub-resolution scales
with strengths between the equipartition field values and 1 kG.

Astrophysics