Tree amplitudes and two-loop counterterms in $D=11\mathrm{}$ supergravity

We compute the tree level 4-particle bosonic scattering amplitudes in $D=11\mathrm{}$ supergravity. By construction, they are part of a linearized supersymmetry, coordinate, and 3-form gauge invariant. While this on-shell invariant is nonlocal, suitable SUSY-preserving differentiations turn it into a local one with correct dimension to provide a natural lowest (two-loop) order counterterm candidate. Its existence shows explicitly that no symmetries protect this ultimate supergravity: Indeed this invariant, together with the recently calculated infinite coefficient of its 4-graviton part, constitutes a demonstration of two-loop non-renormalizability.