  We examine the effects of photon-axion mixing on the CMB. We show that if
there are very underdense regions between us and the last scattering surface
which contain coherent magnetic fields (whose strength can be orders of
magnitude weaker than the current bounds), then photon-axion mixing can induce
observable deviations in the CMB spectrum. Specifically, we show that the
mixing can give rise to non-thermal spots on the CMB sky. As an example we
consider the well known CMB cold spot, which according to the Planck data has a
weak distortion from a black body spectrum, that can be fit by our model. While
this explanation of the non-thermality in the region of the cold spot is quite
intriguing, photon-axion oscillation do not explain the temperature of the cold
spot itself. Nevertheless we demonstrate the possible sensitivity of the CMB to
ultralight axions which could be exploited by observers.
