  In this paper we treat the black hole horizon as a physical boundary to the
spacetime and study its dynamics following from the Gibbons-Hawking-York
boundary term. Using the Kerr black hole as an example we derive an effective
action that describes, in the large wave number limit, a massless Klein-Gordon
field living on the average location of the boundary. Complete solutions can be
found in the small rotation limit of the black hole. The formulation suggests
that the boundary can be treated in the same way as any other matter
contributions. In particular, the angular momentum of the boundary matches
exactly with that of the black hole, suggesting an interesting possibility that
all charges (including the entropy) of the black hole are carried by the
boundary. Using this as input, we derive predictions on the Planck scale
properties of the boundary.
