This really is a very random mine of the day as I snapped it very quickly with my phone as I walked past this evening and I'm afraid it came out a little fuzzy. It is kind of interesting though...
...because this shaft, which leads down to a mine known as Ould Men's Adit, also known as Great Hopewell, is an eighteenth century adit. That means it pre-dates the time when it occurred to people that making some sort of note about where all the mines and adits were before the entire county was hollowed out and collapsing randomly, might be a good idea.
It's enclosed by a Cornish hedge: that's a stone bank mortared with earth and covered in turf, bracken and in this case, gorse and bramble, with a vague later attempt to stop people falling into it by adding a wire fence. The Heritage Gateway says this is one of three shafts, but it is not communicative about where the other shafts might be. There are a couple of other dips along here that might be shafts. Or they might not. Note to self, do not bounce up and down on them just in case.