  We employ a late-time expansion to study the interior of rotating black holes
coupled to a massless scalar field in asymptotically flat spacetime. We find
that decaying fluxes of scalar radiation into the black hole necessitate the
existence of a null singularity at the Cauchy horizon and a central spacelike
singularity at radius $r = 0$. In particular, the decaying influxes source a
localized cloud of scalar radiation near $r=0$ whose amplitude grows
unboundedly large as advanced time $v \to \infty$. The scalar cloud inevitably
results in a central spacelike singularity at late times $v$, with the
curvature near $r = 0$ diverging like $r^{-\alpha v}$, where $\alpha > 0$ is a
constant.
