  We argue that to explain recent resonant tunneling experiments on crystals of
Mn$_{12}$ and Fe$_8$, particularly in the low-T limit, one must invoke dynamic
nuclear spin and dipolar interactions. We show the low-$T$, short-time
relaxation will then have a $\sqrt{t/\tau}$ form, where $\tau $ depends on the
nuclear $T_2$, on the tunneling matrix element $\Delta_{10}$ between the two
lowest levels, and on the initial distribution of internal fields in the
sample, which depends very strongly on sample shape. The results are directly
applicable to the $Fe_8$ system. We also give some results for the long-time
relaxation.
