  We conjecture a topology changing transition in M-theory on a non-compact
asymptotically conical Spin(7) manifold, where a 5-sphere collapses and a CP(2)
bolt grows. We argue that the transition may be understood as the condensation
of M5-branes wrapping the 5-sphere. Upon reduction to ten dimensions, it has a
physical interpretation as a transition of D6-branes lying on calibrated
submanifolds of flat space. In yet another guise, it may be seen as a geometric
transition between two phases of type IIA string theory on a G_2 holonomy
manifold with either wrapped D6-branes, or background Ramond-Ramond flux. This
is the first non-trivial example of a topology changing transition with only
1/16 supersymmetry.
