Lunar Mass Black Holes from QCD Axion Cosmology
  In the QCD axion scenario, a network of domain walls bounded by cosmic
strings fragments into pieces. As these fragments collapse, some of them will
form black holes. With standard QCD axion parameters, the black holes will have
lunar masses ($M_{\rm bh} \sim 10^{-8}\, {\rm M}_\odot$). Even though their
number density is difficult to estimate, arguments suggest that they can
constitute a reasonable fraction of the critical cosmological density.
