  An action principle for spacetimes with the topology of an Euclidean
black-hole is given. The gravitational field is described by the ordinary
volume degrees of freedom plus additional surface fields at the horizon. The
surface degrees of freedom correspond to diffeomorphisms on the sphere at the
horizon and a field of ``opening angles''. General covariance forces the
surface modes to be confined to a box of an unusual exponential shape, whose
volume must be specified as part of the definition of the statistical ensemble.
This gives rise to the Bekenstein-Hawking entropy.
