Near the city of Pontferrada, exists a UNESCO World Heritage Site, Las Médulas. Las Médulas is an outstanding example of innovative Roman technology, in which all the elements of the ancient landscape, both industrial and domestic, have survived to an exceptional degree.
The placer (alluvial) gold deposits of the Las Médulas region were being exploited on a small scale in the late Iron Age. Evidence for this is largely circumstantial, based on excavations of the defended sites (castros) of the region and the related cemeteries, with their wealth of golden objects.
In the 1st century A.D. the Roman Imperial authorities began to exploit the gold deposits of this region in north-west Spain, using a technique based on hydraulic power. Water from springs, rain, and melting snow was collected in large reservoirs, which led by a system of well built gravity canals to the mines themselves, over long distances. Galleries were cut into the sterile strata many metres deep that overlay the layers of auriferous conglomerate. When the sluices of the dams were opened, enormous quantities of water flowed into the galleries, which were closed at their ends. The pressure thus built up caused the rock to explode and to be washed away by the water flow, forming enormous areas of tailings, several kilometres in length. The process is vividly apparent on the working face at the main Las Médulas site. The operating face of this spectacular form of mining slowly moved across the landscape. The system of water canals and conduits has been traced over large areas of the site, and measures at least 100 km.
To quote Pliny the Elder, a procurator in the region in 74 AD, this project was far beyond the work of giants.
Contrary to general belief and unlike the situation in other imperial gold-mining areas (such as Wales), the workers in the mines were free men, not slaves. Their settlements can be found all over the region, alongside yet clearly distinguishable from those which housed the imperial officials and their staffs. Engineering activities, such as the major hydraulic works of building dams and cutting channels and road construction, were the responsibility of the Roman army. The military presence was also maintained to keep the peace and to ensure the safety of imperial officials and their deliveries of gold to provincial capitals and over the sea to Rome.
Sweeping changes took place in the Roman monetary system in the later 2nd century AD, when the gold aureus was devalued, with catastrophic results, not least for the Spanish mines. Caracalla (188-217) restored the aureus to its former place, and as a result the Spanish mines, which had been in crisis, reactivated their production. This may well also explain why Asturia and Callaecia were raised to the status of an independent province, Hispania Nova Citerior Antoniniana. However, both the new province and the resuscitation of the mines seem to have been short-lived, and the lack of later material in the archaeological record shows that gold production effectively came to an end in the opening decades of the 3rd century.
Las Médulas are whats left of what was once the most important gold mine in the Roman Empire.