I had no idea my garden is hollow! I think there may be more lurking underneath. Somewhere landward of the mill race as indicated on the map, small holes appear in my lawn, that look like the topsoil is falling through gaps between the slate it is lying on. Maybe that's the branch going off to the left! Luckily the mill race looks structurally sound. It's quite exciting to have my garden straight on top of such industrial heritage!
A black hole! What's this?
Above it is my garden
What it looks like on the inside
The mill race (or rather, by the looks of it, only the above-ground part of it) on the map. My house (and that of the neighbour - they're connected) is the J-shaped one to the left of the slate yard