NOTE SPECIAL TIME AND LOCATION: IHES, Marilyn and James Simons hall. We show that very heavy pure states in gravity can appear to be mixed states to almost all probes. For half-BPS states, typical states are described by a spacetime ``foam,'' the precise details of which are invisible to almost all probes. We indicate how coarse-graining over Planck scale fluctuations yields semiclassical geometries and propose this as a mechanism for the origin of gravitational thermodynamics. By working in perturbation theory with a small non-extremality parameter, lessons from the supersymmetric situation can be adapted to the microscopics of an almost extremal superstar. We match the counting of the black hole entropy in semiclassical gravity to the degeneracy of states in the dual field theory. Moreover, in certain decoupling limits, the entropy is explained in terms of strings stretching between D-branes.