An M-theory Flop as a Large N Duality
  We show how a recently proposed large $N$ duality in the context of type IIA strings with ${\cal N}=1$ supersymmetry in 4 dimensions can be derived from purely geometric considerations by embedding type IIA strings in M-theory. The phase structure of M-theory on $G_2$ holonomy manifolds and an $S^3$ flop are the key ingredients in this derivation. 
M. Atiyah, J. Maldacena and C. Vafa