  Anatase is a metastable polymorph of TiO$_2$. In contrast to the more
widely-studied TiO$_2$ rutile, O vacancies (V$_\mathrm O$'s) are not stable at
the anatase (101) surface. Low-temperature STM shows that surface V$_\mathrm
O$'s, created by electron bombardment at 105 K, start migrating to subsurface
sites at temperatures $\geq$ 200 K. After an initial decrease of the V$_\mathrm
O$ density, a temperature-dependent dynamic equilibrium is established where
V$_\mathrm O$'s move to subsurface sites and back again, as seen in time-lapse
STM images. We estimate that activation energies for subsurface migration lie
between 0.6 and 1.2 eV; in comparison, DFT calculations predict a barrier of
ca. 0.75 eV. The wide scatter of the experimental values might be attributed to
inhomogeneously-distributed subsurface defects in the reduced sample.
