  We compute the three-loop scattering amplitude of four gravitons in
${\mathcal N}=8$ supergravity. Our results are analytic formulae for a Laurent
expansion of the amplitude in the regulator of dimensional regularisation. The
coefficients of this series are closed formulae in terms of well-established
harmonic poly-logarithms. Our results display a remarkable degree of simplicity
and represent an important stepping stone in the exploration of the structure
of scattering amplitudes. In particular, we observe that to this loop order the
four graviton amplitude is given by uniform weight $2L$ functions, where $L$ is
the loop order.
