  On the basis of the Kerr metric as a model for a spinning black hole
accreting test particles from rest at infinity, I show that the center-of-mass
energy for a pair of colliding particles is generically divergent at the inner
horizon. This shows that not only are classical black holes internally
unstable, but also that Planck-scale physics is a characteristic feature within
black holes at scales much larger that the Planck length. The novel feature of
the divergence discussed here is that the phenomenon is present only for black
holes with rotation and in this sense it is distinct from the well known Cauchy
horizon instability.
