Classical Stability and Quantum Instability of Black-Hole Cauchy Horizons

For a certain region of the parameter space M, e, , the Cauchy horizon of a (charged) black hole residing in de Sitter space is classically stable to gravitational perturbations. This implies that, when left to its own devices, classical theory is unable to retain full predictive power: The evolution of physical fields beyond the Cauchy horizon is not uniquely determined by the initial conditions. In this Letter we argue that the Cauchy horizon of a Reissner--Nordstr\"om--de Sitter black hole must always be unstable quantum mechanically.