The Inner Horizon of a Black Hole

Everyone has, by now, heard of black holes. Astronomically, they are likely to have been supermassive stars that exploded, whose remnant gravitationally collapses to a size the is smaller than a function of its mass called the Schwarzschild radius,

where M is the mass of the star. Once collapse passes this point the star is gone and it becomes a new thing, a black hole. Within the Schwarschild radius nothing can escape from the black hole, the escape velocity grows to be more than the speed of light. There is a boundary that we call the event horizon at the Schwarzschild radius, this is what separates the normal spacetime from the weirdness that happens near the black hole.

There are circumstances that can form another surface inside the event horizon. A charged black hole (Riessner-Nordstrøm) can form an inner horizon caused by a charge barrier. A spinning black hole (Kerr) can form an inner horizon due to transfer of angular momentum.

All horizons are places where, in effect, time stops with respect to an outside observer. The inner horizon must also be traversed by anything that falls into the black hole on its way to the singularity predicted to be at its center. The inner horizon gives the possibility of avoiding the singularity, but there seems no way to gaurantee a trajectory through the black hole to avoid the inner horizon prior to crossing the event horizon (you could still cross the inner horizon).

Even if you avoided the inner horizon, it is unlikely that anything other than subatomic particles will survive the experience.