  We demonstrate the existence of a class of N=1 supersymmetric nonperturbative
vacua of Horava-Witten M-theory compactified on a torus fibered Calabi-Yau
3-fold Z with first homotopy group \pi_{1}(Z)= Z2, having the following
properties: 1) SO(10) grand unification group, 2) net number of three
generations of chiral fermions in the observable sector, and 3) potentially
viable matter Yukawa couplings. These vacua correspond to semistable
holomorphic vector bundles V_{Z} over Z having structure group SU(4)_C, and
generically contain M5-branes in the bulk space. The nontrivial first homotopy
group allows Wilson line breaking of the SO(10) symmetry. Additionally, we
propose how the 11-dimensional Horava-Witten M-theory framework may be used to
extend the perturbative calculation of the top quark Yukawa coupling in the
realistic free-fermionic models to the nonperturbative regime. The basic
argument being that the relevant coupling couples twisted-twisted-untwisted
states and can be calculated at the level of the Z2 X Z2 orbifold without
resorting to the full three generation models.
