Extremal limits and black hole entropy

Taking the extremal limit of a non-extremal Reissner-Nordstr"om black hole (by externally varying the mass or charge), the region between the inner and outer event horizons experiences an interesting fate – while this region is absent in the extremal case, it does not disappear in the extremal limit but rather approaches a patch of $Ad{S}_2\times S^2$. In other words, the approach to extremality is not continuous, as the non-extremal Reissner-Nordstr"om solution splits into two spacetimes at extremality: an extremal black hole and a disconnected $AdS$ space. We suggest that the unusual nature of this limit may help in understanding the entropy of extremal black holes.