The grand unification energy
The unification of the electroweak forces and the strong force with the gravitational force in a so-called "Theory of Everything" requires an even higher energy level which is generally assumed to be close to the Planck Scale. In theory, at such short distances, gravity becomes comparable in strength to the other three forces of nature known to date. This statement is modified if there exist additional dimensions of space at intermediate scales. In this case, the strength of gravitational interactions increases faster at smaller distances, and the energy scale at which all known forces of nature unify, can be considerably lower. This effect is exploited in models of large extra dimensions.
The exact value of the grand unification energy (if grand unification is indeed realized in nature) depends on the precise physics present at shorter distance scales not yet explored by experiments. If one assumes the Desert and supersymmetry, it is at around 1016 GeV.
The most powerful collider to date, the LHC, is designed to reach a center of mass energy of 1.4x104 GeV in proton-proton collisions. The scale 1016 GeV is only a few orders of magnitude below the Planck scale, and thus not within reach of man-made earth bound colliders.