a catastrophe. Instead of settling down to some equilibrium state, a new spacelike singularity forms that extends far outside of what one would naively have called the black hole, reaching out to engulf the entire brane and, in a certain sense, the entire universe.

One is led to wonder whether this behavior is somehow typical or whether other sorts of branes are better behaved. In particular, a priori this might be an artifact of our low-dimensional setting, and a higher dimensional study is currently in progress in collaboration with Joel Rozowsky, Pedro Silva, and Mark Trodden. However, one suspects that the above catastrophe is somehow related to the more basic issue involving black hole thermodynamics. It seems likely that negative tension objects should be allowed only in the case that they have some property that guarantees compatibility with black hole thermodynamics.

This is where we come to the connection with six-branes. We begin by recalling that certain negative tension orientifolds do in fact arise in string theory (see, e.g., [@OGS; @Hanany:2000fq] for recent reviews). String theoretic calculations do not indicate any instabilities of such objects, and given the history of string theory one might well suspect that such objects could 