  The pattern of duality symmetries acting on the states of compactified
superstring models reinforces an earlier suggestion that the Monster sporadic
group is a hidden symmetry for superstring models. This in turn points to a
supersymmetric theory of self-dual and anti-self-dual K3 manifolds joined by
Dirac strings and evolving in a 13 dimensional spacetime as the fundamental
theory. In addition to the usual graviton and dilaton this theory contains
matter-like degrees of freedom resembling the massless states of the heterotic
string, thus providing a completely geometric interpretation for ordinary
matter.
