  The spin relaxation in the molecular magnet \Mn12 is investigated by a novel
type of high-frequency magnetic spectroscopy. By reversing the external
magnetic field the intensities of the $|\pm 10>\to |\pm 9>$ transitions within
the ground multiplet redistribute; below $T\approx 2$ K resonant quantum
tunneling is directly observed as {\em magnetic hole burning} in the absorption
spectra of a polycrystal. We quantitatively describe our observations by taking
into account inhomogeneous line broadening, the time dependence of the level
population during the relaxation process, and the resonance behavior of the
relaxation rate due to quantum tunneling near the avoided level crossing.
