  Low-energy, near-horizon scaling limits of black holes which lead to string
theory on AdS_2 x S^2 are described. Unlike the higher-dimensional cases, in
the simplest approach all finite-energy excitations of AdS_2 x S^2 are
suppressed. Surviving zero-energy configurations are described. These can
include tree-like structures in which the AdS_2 x S^2 throat branches as the
horizon is approached, as well as disconnected AdS_2 x S^2 universes. In
principle, the black hole entropy counts the quantum ground states on the
moduli space of such configurations. In a nonsupersymmetric context AdS_D for
general D can be unstable against instanton-mediated fragmentation into
disconnected universes. Several examples are given.
