Short answers, pulled from the story.
Large-scale production began in the second half of the first century AD after Emperor Augustus brought Hispania Tarraconensis under Roman control in 25 BC. The Romans established this province as their most important gold source and transformed small tribal gatherings into an industrial operation spanning centuries.
Engineers used interbasin transfer to move water across great distances and altitudes to undermine entire mountains with large quantities of flowing water. This technique washed extensive alluvial gold deposits through channels built directly into rock faces while miners worked by lamp light inside galleries driven for long distances.
Sixty thousand free workers labored at Las Médulas over two centuries yielding a total extraction of five million Roman pounds or one million six hundred forty thousand kilograms. Each year twenty thousand Roman pounds produced approximately six thousand five hundred sixty kilograms of gold before opencast work became uneconomical.
Massive smelting activities released heavy metals into the atmosphere above the mining region causing mineral air pollution that was not matched again until the Industrial Revolution some 1,700 years later. Ice core data taken from Greenland confirm these atmospheric lead levels peaked during the Roman period due to the scale of destruction caused by the operations.
Systematic archaeological studies began in 1988 under the Spanish Council for Scientific Research group with new teams mapping settlements across the area. Advanced aerial surveys using LIDAR technology confirmed wide extents of Roman-era works in 2014 while excavations revealed pre-Roman and Roman communities throughout the region surrounding Las Médulas.