  We compute the symbol of the two-loop five-point amplitude in $\mathcal N=8$
supergravity. We write an ansatz for the amplitude whose rational prefactors
are based on not only 4-dimensional leading singularities, but also
$d$-dimensional ones, as the former are insufficient. Our novel $d$-dimensional
unitarity-based approach to the systematic construction of an amplitude's
rational structures is likely to have broader applications, for example to
analogous QCD calculations. We fix parameters in the ansatz by performing
numerical integration-by-parts reduction of the known integrand. We find that
the two-loop five-point $\mathcal N=8$ supergravity amplitude is uniformly
transcendental. We then verify the soft and collinear limits of the amplitude.
There is considerable similarity with the corresponding amplitude for $\mathcal
N=4$ super-Yang-Mills theory: all the rational prefactors are double copies of
the Yang-Mills ones and the transcendental functions overlap to a large degree.
As a byproduct, we find new relations between color-ordered loop amplitudes in
$\mathcal N=4$ super-Yang-Mills theory.
