  We show that there exists an exponentially large discretuum of vacua in
$G_2$-compactifications of M-theory without flux. In M-theory-inspired
$G_2$-MSSM, quantities relevant for particle physics remain virtually
insensitive to large variations of the vacuum energy across the landscape. The
purely non-perturbative vacua form a special subset of a more general class of
vacua containing fractional Chern-Simons contributions. The cosmological
constant can be dynamically neutralized via a chain of transitions interpolated
by fractional gauge instantons describing spontaneous nucleation of M2-brane
domain walls. Each transition is generically accompanied by a gauge symmetry
breaking in some sector of the theory. In particular, the visible sector GUT
symmetry breaking can likewise be triggered by a spontaneous nucleation of an
M2-brane.
