  We study the physics of singular limits of $G_2$ compactifications of
M-theory, which are necessary to obtain a compactification with non-abelian
gauge symmetry or massless charged particles. This is more difficult than for
Calabi-Yau compactifications, due to the absence of calibrated two-cycles that
would have allowed for direct control of W-boson masses as a function of
moduli. Instead, we study the relationship between gauge enhancement and
singular limits in $G_2$ moduli space where an associative or coassociative
submanifold shrinks to zero size; this involves the physics of topological
defects and sometimes gives indirect control over particle masses, even though
they are not BPS. We show how a lemma of Joyce associates the class of a
three-cycle to any $U(1)$ gauge theory in a smooth $G_2$ compactification. If
there is an appropriate associative submanifold in this class then in the limit
of nonabelian gauge symmetry it may be interpreted as a gauge theory
worldvolume and provides the location of the singularities associated with
non-abelian gauge or matter fields. We identify a number of gauge enhancement
scenarios related to calibrated submanifolds, including Coulomb branches and
non-isolated conifolds, and also study examples that realize them.
