  In order to gain insight into the possible Ground State of Quantized
Einstein's Gravity, we have devised a variational calculation of the energy of
the quantum gravitational field in an open space, as measured by an asymptotic
observer living in an asymptotically flat space-time. We find that for Quantum
Gravity (QG) it is energetically favourable to perform its quantum fluctuations
not upon flat space-time but around a ``gas'' of wormholes, whose size is the
Planck length $a_p$ ($a_p\simeq 10^{-33}$cm). As a result, assuming such
configuration to be a good approximation to the true Ground State of Quantum
Gravity, space-time, the arena of physical reality, turns out to be well
described by Wheeler's Quantum Foam and adequately modeled by a space-time
lattice with lattice constant $a_p$, the Planck lattice.
