  Recent observations have positioned the endpoint of the Epoch of Reionisation
(EoR) at redshift $z \sim 5.3$. However, observations of the Lyman-$\alpha$
forest have not yet been able to discern whether reionisation occurred slowly
and late, with substantial neutral hydrogen persisting at redshift $\sim 6$, or
rapidly and earlier, with the apparent late end driven by the fluctuating UV
background. Gunn-Peterson (GP) absorption troughs are solid indicators that
reionisation is not complete until $z=5.3$, but whether they contain
significantly neutral gas has not yet been proven. We aim to answer this
question by directly measuring, for the first time, the neutral hydrogen
fraction ($x_\mathrm{HI}$) at the end of the EoR ($5 \lesssim z \lesssim 6$) in
high-redshift quasars spectra. For high neutral fractions
$x_\mathrm{HI}\gtrsim0.1$, GP troughs exhibit damping wing (DW) absorption
extending over $1000$ km s$^{-1}$ beyond the troughs. While conclusively
detected in Lyman-$\alpha$ emission lines of quasars at $z\geq7$, DWs are
challenging to observe in the general Lyman-$\alpha$ forest due to absorption
complexities and small-scale stochastic transmission features. We report the
first successful identification of the stochastic DW signal adjacent to GP
troughs at redshifts $z=5.6$ through careful stacking of the dark gaps in
Lyman-$\alpha$ forest. We use the signal to present a measurement of the
corresponding global $x_\mathrm{HI}=0.19\pm0.07$ $(_{-0.16}^{+0.11})$ at
$1\sigma$ $(2\sigma)$ at $z=5.6$ and a limit $x_\mathrm{HI}<0.44$ at $z=5.9$.
The detection of this signal demonstrates the existence of substantially
neutral islands near the conclusion of the EoR, unequivocally signaling a
late-and-slow reionization scenario.
