Overgrown mines are really hard to photograph. It's strange: you can see the outlines of quite a lot of the buildings quite clearly under the ivy in real life, but somehow in a photo all you get is a blob even if it's all in focus. According to the Heritage Gateway, there's a chimney for an arsenic calcining plant in here somewhere, but I have yet to spot it. It's probably hiding, disguised as a tree.
Wheal Benny was a smallish tin, wolfram, and yet, of course arsenic mine. It goes right down to the riverbank and had a waterwheel. To the left of Rosie, the wall of a wheelpit, and to her right, the river.
This is probably the most visible bit of it, with the gate across the adit entry to the mine at the back.
I think this was probably a wheel pit for one of the waterwheels. It's quite deep (and completely unfenced: note to self, keep careful track of location of both dogs at this point).