We review recent work concerning topology changing phase transitions t
hrough black hole condensation in Type II string theory. We then also
briefly describe a present study aimed at extending the known web of i
nterconnections between Calabi-Yau manifolds. We show, for instance, t
hat all 7555 Calabi-Yau hypersurfaces in weighted projective four spac
e are mathematically connected by extremal transitions.