Las Médulas is the most spectacular Roman engineering relic in Spain and one of the strangest landscapes in Europe — a moonscape of red sandstone pinnacles, caves and gullies created by hydraulic gold mining on a colossal scale. From the first century AD, Roman engineers undermined whole mountains by flushing them with water diverted through aqueducts from up to 100 kilometres away. When the operation closed, around the third century, what was left was a chaos of crumbled cliffs, now thickly forested with sweet chestnut.
The site is a UNESCO World Heritage Site and the best of it is seen from the Mirador de Orellán, a viewpoint reached by a short walk above the village of Orellán. From there you can pick out the silhouettes of giant pinnacles among the chestnut trees. Trails of varying lengths run through the site itself; the Senda Perimetral, about 7 km, is the classic walk and passes through tunnels driven by Roman engineers two thousand years ago. Come in late October when the chestnut leaves turn red and gold.