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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 newdata 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-sightvariance in the intergalactic Ly$\alpha$ opacity over $4 \le z \le 6$. We findthat the scatter in transmission among lines of sight near $z \sim 6$significantly exceeds theoretical expectations for either a uniform ultravioletbackground (UVB) or simple fluctuating UVB models in which the mean free pathto ionizing photons is spatially invariant. The data, particularly near $z\simeq 5.6$-5.8, instead require fluctuations in the volume-weighted hydrogenneutral fraction that are a factor of 3 or more beyond those expected fromdensity variations alone. We argue that these fluctuations are most likelydriven by large-scale variations in the mean free path, consistent withexpectations for the final stages of inhomogeneous hydrogen reionization. Evenby $z \simeq 5.6$, however, a large fraction of the data are consistent with auniform UVB, and by $z \sim 5$ the data are fully consistent with opacityfluctuations arising solely from the density field. This suggests that whilereionization may be ongoing at $z \sim 6$, it has fully completed by $z \sim5$.

Evidence of patchy hydrogen reionization from an extreme Ly$\alpha$ trough below redshift six