  We explore the cosmic evolution of a scalar field which is identified with
the four dimensional spacetime volume. Given a specific form for the Lagrangian
of the scalar field, a new holographic dark energy model is present. The energy
density of dark energy is reversely proportional to the square of the radius of
the cosmic null hypersurface which is present as a new infrared cutoff for the
Universe. We find this holographic dark energy belongs to the phantom dark
energy for some appropriate parameters in order to interpret the current
acceleration of the Universe.
