  Regarding the volume as independent thermodynamic variable we point out that
black hole horizons can hide positive heat capacity and specific heat. Such
horizons are mechanically marginal, but thermally stable. In the absence of a
canonical volume definition, we consider various suggestions scaling
differently with the horizon radius. Assuming Euler-homogeneity of the entropy,
besides the Hawking temperature, a pressure and a corresponding work term
render the equation of state at the horizon thermally stable for any meaningful
volume concept that scales larger than the horizon area. When considering also
a Stefan--Boltzmann radiation like equation of state at the horizon, only one
possible solution emerges: the Christodoulou--Rovelli volume, scaling as $V\sim
R^5$, with an entropy $S = \frac{8}{3}S_{BH}$.
