I've visited this site several times, Ystrad Einon, a copper mine, is situated near the Artists Valley in West Wales, accessible only via a remote track, the drive to visit is a hairy one, especially when you decide to go in the depths of Winter, which I would not recommend. The first think you come across when you arrive at the site, is a set of crumbling remains of the above ground workings. There are 2 entrances to the underground part that I know of, one vertical shaft, which is more or less inaccessible, and a horizontal adit, which is a short treck through the woods. The adit is usually filled up to the shin with freezing water for the first 20-30 meters, but as it rises, the water becomes shallower until the ground is completely dry. The floor of the passages through the mine are sound, and it is fairly obvious when there is a shaft. The explorable areas of the mine are substantial, but collapses, either purposeful or accidental block a large area of the mine. The main highlight of the mine is a giant water week in a cavernous opening, leaving you wondering how on earth it got into the mine, and how they assembled it.
I don't have that many images, but I'm planning to return to get some soon.