Mass inflation on the Cauchy horizon
(Poisson & Israel 1990)

Ingoing and outgoing fluids, if they drop through the inner horizon, must drop into causally separated regions of spacetime. To do this, they must exceed the speed of light relative to each other, which is impossible.

Feedback loop:
Counter-streaming ingoing and outgoing fluids generate a large proper pressure.
The pressure increases the inward gravitational force,
accelerating the ingoing and outgoing fluids through each other.

Why does gravity accelerate ingoing and outgoing fluids in opposite directions?
Inward  =

towards BH ingoing
away from BH outgoing

Result: Mass inflation

  RN Penrose diagram

  Mass inflation can lead to a null singularity along the Cauchy horizon