teach us something new about gravitational physics. Thus, one expects that such orientifolds are the best candidates for negative tension objects that may be compatible with black hole thermodynamics.

As with the puzzles discussed above, it appears that the six-plane case can shed further light on the issues. It turns out that (at strong coupling) there is an M-theoretic understanding of the orientifold six-plane [@Sei1; @Sei2; @Sen] in terms of the Atiyah-Hitchin manifold [@AH]. The resulting 11-dimensional spacetime is in fact a smooth Ricci-flat (hyperkähler) manifold whose structure near infinity resembles a $Z_2$ quotient of a negative tension Kaluza-Klein monopole. As a result, one may describe the collision of a black hole with such an orientifold as a problem in pure [*vacuum*]{} (i.e., source-free) Einstein-Hilbert gravity in eleven dimensions. Under such conditions, the Raychaudhuri equation leads in the usual way [@HE] to the conclusion that the total horizon area must increase during the collision. Our violations of the generalized second law of thermodynamics will not arise in this context. As the various stringy negative tension orientifolds are related by T-duality, one expects that the other orientifolds of string theory also have properties such that the second law of thermodynamics is 