  A longstanding question in superstring/$M$ theory is does it predict
supersymmetry below the string scale? We formulate and discuss a necessary
condition for this to be true; this is the mathematical conjecture that all
stable, compact Ricci flat manifolds have special holonomy in dimensions below
eleven. Almost equivalent is the proposal that the landscape of all geometric,
stable, string/$M$ theory compactifications to Minkowski spacetime (at leading
order) are supersymmetric. For simply connected manifolds, we collect together
a number of physically relevant mathematical results, emphasising some key
outstanding problems and perhaps less well known results. For non-simply
connected, non-supersymmetric Ricci flat manifolds we demonstrate that many
cases suffer from generalised Witten bubble of nothing instabilities.
