It's the boundary that marks the distance at which light can no longer escape a black hole – leaving us striving for answers about what a black hole actually looks like without a way to see inside. For a Schwarzschild black hole – one that is not rotating – the singularity remained a point. What it might look like orbiting a black hole outside the photon sphere at almost the speed of light. Instead, the authors argue that a realistic Kerr black hole would collapse into a spacelike singular surface at its inner horizon, making its singularity more like that of a Schwarzschild black hole than the familiar textbook picture suggests. Both theories work extraordinarily well on their own, but they become mathematically incompatible under the extreme conditions inside a black hole.