  Our universe may have formed via bubble nucleation in an eternally-inflating
background. Furthermore, the background may have a compact dimension---the
modulus of which tunnels out of a metastable minimum during bubble
nucleation---which subsequently grows to become one of our three large spatial
dimensions. When in this scenario our bubble universe collides with other ones
like it, the collision geometry is constrained by the reduced symmetry of the
tunneling instanton. While the regions affected by such bubble collisions still
appear (to leading order) as disks in an observer's sky, the centers of these
disks all lie on a single great circle, providing a distinct signature of
anisotropic bubble nucleation.
