Imagine a whole mountain range turned inside out. That is Las Médulas, the largest open-pit gold mine in the Roman Empire, worked after the conquest of north-west Spain under Augustus and kept going for around two centuries. The Romans, instead of digging tunnels alone, brought water down from distant mountains through canals that ran for many kilometres, flooded the galleries they had cut into the hill and let the pressure bring the mountain down. Pliny the Elder, who served in Spain, described the method and called it ruina montium, the ruin of mountains.
What was left behind is now one of the strangest landscapes in Spain: red and orange pinnacles of clay and sandstone, rising from a forest of ancient chestnut trees. The chestnuts were planted by the Romans, or at least grew on the spoil, and in October the colours are almost absurd, with the red rock, gold leaves and dark pines. The nearby lake of Carucedo is the dammed remains of a river that the mining waste blocked. The site is protected by UNESCO, and the best view is from the Mirador de Orellán, a short and steep walk up from the road.
Riding here is a quiet affair. The roads into the area are narrow and winding, with farm traffic and the occasional tour bus, and the surfaces in small villages can be rough. Carucedo has only about six hundred people, so for a proper fuel stop and a decent meal, head for Cacabelos or Ponferrada, which lie in the Bierzo valley to the east, famous for its Mencía wine and for the cured meats and peppers of the region.
Come in the morning for softer light, and stay for the sunset if you can, when the cliff faces glow. Bring layers, since the hills can be cold early and late in the year, and keep your speed down in the villages, where chestnut burrs and wet leaves can make the surface slick.