  We study the physics of globally consistent four-dimensional $\mathcal{N}=1$
supersymmetric M-theory compactifications on $G_2$ manifolds constructed via
twisted connected sum; there are now perhaps fifty million examples of these
manifolds. We study a rich example that exhibits $U(1)^3$ gauge symmetry and a
spectrum of massive charged particles that includes a trifundamental. Applying
recent mathematical results to this example, we compute membrane instanton
corrections to the superpotential and spacetime topology change in a compact
model; the latter include both the (non-isolated) $G_2$ flop and conifold
transitions. The conifold transition spontaneously breaks the gauge symmetry to
$U(1)^2$, and associated field theoretic computations of particle charges make
correct predictions for the topology of the deformed $G_2$ manifold. We discuss
physical aspects of the abelian $G_2$ landscape broadly, including aspects of
Higgs and Coulomb branches, membrane instanton corrections, and some general
aspects of topology change.

High Energy Physics - Theory;Differential Geometry