  We consider the problem of determination of a volume of some bounded
space-like hypersurfaces in the case of spherically symmetric spacetimes. In
the case when the hypersurfaces is cut or bounded by a light-like hypersurface
the problem may not be well defined. In order to define properly the volume we
introduce the volume forms related to the given foliation (observer) of the
considered spacetime. In the case of the constant curvature slice the volume of
the hypersurface cut by the horizon (light-like surface) becomes composed of
the two parts, outer and inner, treated differently. We compute the
corresponding volumes outside and inside of the horizon of the ethernal
Schwarzschild black hole.
