A tricky part was making sure that numerical instabilities didn't propagate outward away from the event horizon. I believe there are now several solutions to th

A tricky part was making sure that numerical instabilities didn't propagate outward away from the event horizon. I believe there are now several solutions to the problem, but the one I find most intuitive is simply extending spacetime ad infinitum inside the black hole in order to allow gravitational waves to propagate inward forever. More here: https://en.wikipedia.org/wiki/Numerical_relativity https://en.wikipedia.org/wiki/Numerical_relativity