Apparently this shaft is the deepest of the South Caradon shafts, it's 250 fathoms deep. It's called the sump because it's the point to which the water drained. Apparently the shaft is more than half a kilometre deep and is four times as deep as the distance between the floor of the valley and the peak of the hill.
Wow, that's deep.
A person needs to be careful not to fall down one of these.