  We study M and F theory compactifications on Calabi-Yau four-folds in the
presence of non-trivial background flux. The geometry is warped and belongs to
the class of p-brane metrics. We solve for the explicit warp factor in the
orbifold limit of these compactifications, compare our results to some of the
more familiar recently studied warped scenarios, and discuss the effects on the
low-energy theory. As the warp factor is generated solely by backreaction, we
may use topological arguments to determine the massless spectrum. We perform
the computation for the case where the four-fold equals K3 \times K3.
