Loops from Nodes: Two-loop supergravity amplitudes from the ambitwistor string

The last years have seen remarkable progress in understanding the scattering amplitudes of massless particles in arbitrary dimensions.Underlying the simple formulae are chiral worldsheet models, known as Ambitwistor Strings. While correlators of these models admit a conventional genus expansion of the worldsheet, the amplitudes actually localize on the maximal non-separating degeneration. We explore this simplification at two loops for type II supergravity, concluding in several observations for generic massless field theories.