  We present evidence that a region of high effective Ly$\alpha$ optical depth
at $z\sim5.7$ is associated with an underdense region at the tail end of cosmic
reionization. We carried out a survey of Lyman-break Galaxies (LBGs) using
Subaru Hyper Suprime-Cam in the field of the $z=5.98$ quasar J0148+0600, whose
spectrum presents an unusually long ($\sim160 \mathrm{cMpc}$) and opaque ($\tau
\gtrsim7$) Ly$\alpha$ trough at $5.5\le z\le 5.9$. LBG candidates were selected
to lie within the redshift range of the trough, and the projected number
densities were measured within 90~cMpc of the quasar sightline. The region
within $8'$ (or $\approx 19~\mathrm{cMpc}$) of the quasar position is the most
underdense of the whole field. The significance of the presence of the void is
estimated to be $99\%$. This is consistent with the significant deficit of
Ly$\alpha$ emitters (LAEs) at $z=5.72$ reported by Becker et al. and suggests
that the paucity of LAEs is not purely due to the removal of the Ly$\alpha$
emission by the high opacity but reflects a real coherent underdensity of
galaxies across the entire redshift range of the trough. These observations are
consistent with scenarios in which large optical depth fluctuations arise due
to fluctuations in the galaxy-dominant UV background or due to residual neutral
islands that are expected from reionization that is completed at redshifts as
low as $z\lesssim5.5$.
