  The entanglement entropy of the Hawking radiation contains contributions from
a region inside the black hole, which is called islands, implying that the
Hawking radiation contains the information of islands. The boundary of the
island is given by the quantum extremal surface, whose position is determined
so that the entanglement entropy is extremized. In many cases of stationary
black holes and a few cases of evaporating black holes, it was already
confirmed that the quantum extremal surface is located outside the horizon for
stationary black holes and is inside the horizon for evaporating black holes.
In this paper, we calculate islands in general black holes and show that the
island extends to the outside of the horizon for stationary black holes but is
hidden inside the horizon for evaporating black holes independent of details of
the black hole.
