  String theory/M-theory generally predicts that axionic fields with a broad
mass spectrum extending below 10^{-10}eV are produced after compactification to
four dimensions. These axions/fields provoke a rich variety of cosmophysical
phenomena on different scales depending on their masses and provide us new
windows to probe the ultimate theory. In this article, after overviewing this
axiverse idea, I take up the black hole instability as the most fascinating one
among such axionic phenomena and explain its physical mechanism and
astrophysical predictions.
