Evidence of patchy hydrogen reionization from an extreme Ly$\alpha$
  trough below redshift six
  We report the discovery of an extremely long ($\sim$110 Mpc/$h$) and dark
($\tau_{\rm eff} \gtrsim 7$) Ly$\alpha$ trough extending down to $z \simeq 5.5$
towards the $z_{\rm em} \simeq 6.0$ quasar ULAS J0148+0600. We use these new
data in combination with Ly$\alpha$ forest measurements from 42 quasars at $4.5
\le z_{\rm em} \le 6.4$ to conduct an updated analysis of the line-of-sight
variance in the intergalactic Ly$\alpha$ opacity over $4 \le z \le 6$. We find
that the scatter in transmission among lines of sight near $z \sim 6$
significantly exceeds theoretical expectations for either a uniform ultraviolet
background (UVB) or simple fluctuating UVB models in which the mean free path
to ionizing photons is spatially invariant. The data, particularly near $z
\simeq 5.6$-5.8, instead require fluctuations in the volume-weighted hydrogen
neutral fraction that are a factor of 3 or more beyond those expected from
density variations alone. We argue that these fluctuations are most likely
driven by large-scale variations in the mean free path, consistent with
expectations for the final stages of inhomogeneous hydrogen reionization. Even
by $z \simeq 5.6$, however, a large fraction of the data are consistent with a
uniform UVB, and by $z \sim 5$ the data are fully consistent with opacity
fluctuations arising solely from the density field. This suggests that while
reionization may be ongoing at $z \sim 6$, it has fully completed by $z \sim
5$.
