  It has been recently revealed that strontium titanate (SrTiO$_3$) displays
persistent photoconductivity with unique characteristics: it occurs at room
temperature and lasts over a very long period of time. Illumination of
SrTiO$_3$ crystals at room temperature with sub-bandgap light reduces the
electrical resistance by three orders of magnitude and persists for weeks or
longer (Tarun ${\it et\: al.}$, Phys. Rev. Lett. ${\bf 111}$, 187403 (2013)).
Experiments indicate that oxygen vacancy and hydrogen play important roles, yet
the microscopic mechanism responsible for this remarkable effect has remained
unidentified. Using hybrid density functional theory calculations we show that
an instability associated with substitutional hydrogen H$_{\rm O}^+$ under
illumination, which becomes doubly ionized and leaves the oxygen site, can
explain the experimental observations. H$_{\rm O}$ then turns into an
interstitial hydrogen and an oxygen vacancy, leading to excess carriers in the
conduction band. This phenomenon is not exclusive to SrTiO$_3$, but it is also
predicted to occur in other oxides. Interestingly, this phenomenon represents
an elegant way of proving the existence of hydrogen substituting on an oxygen
site (H$_{\rm O}$), forming an interesting, and rarely observed, type of
three-center two-electron bond.

Materials Science