This is quite obscure, but is a large 600' long adit with a couple of crosscuts to a vein below a blocked shaft. The entrance is improbably situated, and although thousands of people pass near it every year, I doubt any have seen it.
It is coastal. In fact maybe 5m above the sea. It's in an area with one unrelated mega-mine, although once you start digging on maps, you'll find trials, shafts and adits all over the area.