You're not far from the quarry at pindale but my money's on it being to do with the cave tours (the devil's rrsse) I had the pleasure of going on the tour in the summer and highly recommend it
take warm clothes though as even in the middle of summer it was quite cold down there - come to think of it, one of the guides said there's another shaft that they've been unable to find but some local cavers thought they might have found it and were working on getting into it, perhaps it's to do with that