The ancient Roman goldmines at Las Médulas, about 20km southwest of Ponferrada, once served as the main source of gold for the entire Roman Empire – the final tally came to a remarkable 3 million kilograms. It's stunningly beautiful, especially at sunset – one of the more bizarre landscapes you'll see in Spain. The best views are from the Mirador de Orellán, while there are some terrific walks from the village of Las Médulas.
Las Médulas' otherworldly aspect is not a natural phenomenon: an army of slaves honeycombed the area with canals and tunnels (some over 40km long), through which they pumped water to break up the rock and free it from the precious metal.
To get here, take the N536 southwest of Ponferrada, then take signed turn-off in the village of Carucedo. On the outskirts of Carucedo, the road forks. The left fork leads to the pretty stone village of Orellán (2.8km on), which tumbles down into a valley, and then on to the car park for the Mirador de Orellán. From the car park it's a steep 750m climb up to the lookout, from where the views are breathtaking.
Returning to the fork, the right branch leads on to Las Médulas village (3km on). Park at the entrance to the village, then stop at at the Aula Arqueológica, an interpretation centre with interesting displays on the history of the mines (admission €2); it also has information on the walks that weave among chestnut trees to the caves, quarries and bizarre formations left behind by the miners. If you're feeling fit, a 5.1km trail climbs to the Mirador de Orellán (one way/return three/4½ hours) from here, but there are plenty of other shorter trails into the heart of Las Médulas. You can also enquire about horse riding in the area.
Las Médulas can get overwhelmed with visitors on weekends, but is much quieter during the week. Las Médulas village has a handful of restaurants and casas rurales in case you're tempted to linger.