From an interesting article in Science Magazine – Trio of big black holes spotted in galaxy smashup, one sentence jumped out at me…
…three supermassive black holes orbiting close to one another, two of them just a few hundred light-years apart.
Golly – JUST a few hundred light-years apart.
Otherwise the item is interesting because this would be a great source of gravitational waves. Harnessing a small or quantum singularity and vibrating that may be more useful, but until we figure that out, we can watch these supermassive babies.