Short answers, pulled from the story.
Las Médulas is a historic gold-mining site near the town of Ponferrada in the comarca of El Bierzo, in northwestern Spain. It was the most important gold mine and the largest open-pit gold mine in the Roman Empire. UNESCO added it to the World Heritage list in 1997, recognizing it as both an engineering legacy and a cultural landscape.
Ruina montium, meaning the wrecking of the mountains, was a Roman hydraulic mining technique that involved driving tunnels beneath hillsides and flooding them with water until the hillsides collapsed. At Las Médulas, water was delivered by at least seven long aqueducts drawing from the La Cabrera district at different altitudes. Pliny the Elder described the method in 77 AD in his Naturalis Historia.
Over 250 years, Las Médulas produced 5 million Roman pounds of gold, roughly 1.64 million kilograms. Pliny the Elder recorded the mine's annual yield at 20,000 Roman pounds, the equivalent of 6,560 kilograms. Sixty thousand free workers drove the operation.
Las Médulas and other Roman mining sites produced atmospheric lead pollution at levels not matched again until the Industrial Revolution, some 1,700 years later. Ice core data from Greenland suggests that mineral air pollution peaked during the Roman period. The scale of environmental damage led Thailand's delegate to formally oppose the site's UNESCO World Heritage nomination.
Las Médulas was added to the UNESCO World Heritage list in 1997. The designation followed systematic archaeological research underway since 1988, carried out by a group within Spain's Council for Scientific Research, known as CSIC. That research reframed the site as a cultural landscape rather than simply a historic mine.
Pliny the Elder was a Roman official who served as a procurator in the Las Médulas region in 74 AD. In his Naturalis Historia, he described the ruina montium mining technique and the underground galleries called arrugiae, noting that miners worked by lamplight for months without seeing daylight. He wrote that the scale of the enterprise outdid even the achievements of the Giants.