The ‘old’ Harecastle Canal Tunnel, the southern end of which you see here, is no longer in use, even though it was used extensively in the nineteenth century together with the ‘new’ Harecastle Tunnel which it sits alongside. (One tunnel used to take traffic in one direction, the other in the other).
However, it started to subside, and that was that.
The curious red stain in the water is because there is iron ore in the hill above the tunnel, and traces of the ore seep in. The reeds are supposed to filter the colouring out.