  We find the first binary black hole event horizon with a toroidal topology.
It had been predicted that generically the event horizons of merging black
holes should briefly have a toroidal topology, but such a phase has never been
seen prior to this work. In all previous binary black hole simulations, in the
coordinate slicing used to evolve the black holes, the topology of the event
horizon transitions directly from two spheres during the inspiral to a single
sphere as the black holes merge. We present a coordinate transformation to a
foliation of spacelike hypersurfaces that "cut a hole" through the event
horizon surface, resulting in a toroidal event horizon. A torus could
potentially provide a mechanism for violating topological censorship. However,
these toroidal event horizons satisfy topological censorship by construction,
because we can always trivially apply the inverse coordinate transformation to
remove the topological feature.
