  Long-lived population storage in optically pumped levels of rare-earth ions
doped into solids, referred to as persistent spectral hole burning, is of
significant fundamental and technological interest. However, the demonstration
of deep and persistent holes in rare-earth ion doped amorphous hosts, e.g.
glasses, has remained an open challenge since many decades -- a fact that
motivates our work towards a better understanding of the interaction between
impurities and vibrational modes in glasses. Here we report the first
observation and detailed characterization of such holes in an erbium-doped
silica glass fiber cooled to below 1 K. We demonstrate population storage in
electronic Zeeman-sublevels of the erbium ground state with lifetimes up to 30
seconds and 80\% spin polarization. In addition to its fundamental aspect, our
investigation reveals a potential technological application of rare-earth ion
doped amorphous materials, including at telecommunication wavelength.
