  We examine brane induced gravity on codimension-1 branes, a.k.a DGP gravity,
as a theory of five-dimensional gravity containing a certain class
four-dimensional branes. From this perspective, the model suffers from a number
of pathologies which went unnoticed before. By generalizing the 5D geometry
from Minkowski to Schwarzschild, we find that when the bulk mass is large
enough, the brane hits a pressure singularity at finite radius. Further, on the
self-accelerating branch, the five-dimensional energy is unbounded from below,
implying that the self-accelerating backgrounds are unstable. Even in an empty
Minkowski bulk, standard Euclidean techniques suggest that the spontaneous
nucleation of self-accelerating branes is unsuppressed. If so, quantum effects
will strongly modify any classical intuition about the theory. We also note
that unless considered as Z_2-orbifold boundaries, self-accelerating branes
correspond to `wormhole' configurations, which introduces the usual problematic
issues associated with wormholes. Altogether these pathologies present a
serious challenge that any proposed UV completion of the DGP model must
overcome.
