Las Médulas is the largest open-pit gold mine from the Roman Empire. Operational between the 1st and 3rd century AD, water pressure was used to extract the gold.
Fresh water from nearby mountain lakes (at a higher altitude) was directed through canals to reservoirs constructed on top of the gold-bearing hills. Beneath the reservoirs, channels were dug into the light-soil hills. Upon releasing the reservoir sluice gates, the hydrostatic pressure from the descending water smashed open the hills, washing the deposits of gold downhill for panning by hundreds of workers. What is left are the remains of the hills, appearing as reddish-orange peaks.
Las Medulas produced around 1.5 million kilograms of gold over two hundred years.