  Recent experimental observations of unexpectedly large thermal Hall
conductivities, $\kappa_H$, in insulating materials, including the parent
compounds of the high temperature superconducting cuprates, likely reflect an
extrinsic contribution from a yet to be identified extrinsic source of skew
scattering of acoustic phonons. We show that resonant scattering of phonons
from a certain class of three-level systems produces strong skew scattering in
the presence of a modest magnetic field. We interpret this as a first step
towards understanding the experiments.
