  We report Hall sensor measurements of the magnetic relaxation of
Mn$_{12}$-acetate as a function of magnetic field applied along the easy axis
of magnetization for a series of closely-spaced temperatures between 0.24 K and
1.9 K. We map out a region in the $H-T$ plane where ground state tunneling is
observed, a region where tunneling proceeds via thermally-assisted tunneling,
and a crossover region where both participate in the relaxation. We observe the
occasional absence of ground-state tunneling under conditions where one would
expect it to be present, and suggest a resolution to the enigma.
