  The Standard Model data suggests that the quantum gravity vacuum should
accommodate two pivotal ingredients. The existence of three chiral generations
and their embedding in chiral 16 SO(10) representations. The Z2XZ2 orbifolds
are examples of perturbative heterotic string vacua that yield these
properties. The exploration of these models in the nonperturbative framework of
M-theory is discussed. A common prediction of these contructions is the
existence of super-heavy meta-stable states due to the Wilson-line breaking of
the GUT symmetries. Cosmic ray experiments in the forthcoming years offer an
exciting experimental window to the phenomenology of such states.
