Black holes are real. We see them throughout the cosmos, and have even directly imaged the supermassive black hole in M87 and our own Milky Way. We understand black holes quite well, but the theoretical descriptions of these cosmic creatures still have nagging issues. Perhaps the most famous issue is that of the singularity. According to the classical model of general relativity, all the matter that forms a black hole must be compressed into an infinite density, enclosed within a sphere of zero volume. We assume that somehow quantum physics will avert this problem, though without a theory of quantum gravity, we aren’t sure how. But the singularity isn’t the only infinite problem. Take, for example, the strange boundary known as the Cauchy horizon.