A wonderful place to visit, right on the edge of the Montanes de Leon, not far from Ponferrada. The site was recommended by a Portuguese colleague, as worth driving some miles for, and we agree with his assessment.
Go to the village of Las Medulas first, go to the museum, where they explain how the mining of the entire mountain worked using just the pressure of water. The remains of the mining landscape covers a large area with many walking trails though some great terrain with weird ridges of remaining orange snadstones. We took a walk up to the Caves and then headed back to the village which was a nice 1-2 hour walk around the site. The village has many different small cafes and eating places (except on a Fete day!). Try the local honey, with it's rich taste of the Chestnut flowers. Sample some of the locally produced jams including Quince Marmalade. The local ladies will explain how they take the bee products and use them.
In the heat of the summer, an afternoon visit will be challenging due to the heat, but in September it was fine. We finished off with a great meal sampling some of the local dishes in the Hotel at Las Medulas, with a terrace overlooking the lake.
Then at the end it's well worth driving up to the view point and looking down on the remnants of the mined mountain, with fabulous Chestnut woods, when you can see the scale of the Roman mining efforts. It's a worthy World Heritage site!
Probably best done on a good day, as you need good visibility to appreciate the scale of the mining.