5-Kerr Black Hole
The type of black hole most people are familiar with, which has an event horizon on the outside acting as the “point of no return” and a point singularity of infinite density on the inside, actually has a more specific name: a Schwarzschild black hole.
It is named after Karl Schwarzschild, who found the mathematical solution of Einstein’s field equations for a spherical, non-rotating mass in 1915, only a month after Einstein actually published his general theory of relativity. However, it wasn’t until 1963 that mathematician Roy Kerr found the solution for a rotating spherical mass. Hence, a rotating black hole is called a Kerr black hole, and it has some unusual properties.
At the centre of a Kerr black hole, there is no point singularity, but rather a ring singularity—a spinning one-dimensional ring held open by its own momentum. There are also two event horizons, an inner and outer one, and an ellipsoid called the ergosphere, inside which space-time itself rotates with the black hole (because of frame dragging) faster than the speed of light. When entering the black hole, by passing through the outer event horizon, space-like paths become time-like, meaning that it is impossible to avoid the singularity at the centre, just like in a Schwarzschild black hole. However, when you pass through the inner event horizon, your path becomes space-like again.
The difference is this: space-time itself is reversed. This means gravity near the ring singularity becomes repulsive, actually pushing you away from the centre. In fact, unless you enter the black hole exactly on the equator, it is impossible to hit the ring singularity itself. Additionally, ring singularities can be linked through space-time, so they can act as wormholes, although exiting the black hole on the other side would be impossible (unless it was a naked singularity, possibly created when the ring singularity spins fast enough). Traveling through a ring singularity might take you to another point in space-time, such as another universe, where you could see light falling in from outside the black hole, but not leave the black hole itself.
It might even take you to a “white hole” in a negative universe, the exact meaning of which is unknown.