Las Médulas, located near the town of Ponferrada in León province, Spain, used to be the most important gold mine in the Roman Empire. Las Médulas is listed by the UNESCO as one of the World Heritage Sites. The contrast between green of chestnut and oaks with the red color of the mountains in Las Médulas is wonderful.
How were Las Médulas made? The spectacular landscape of Las Médulas resulted from the Ruina Montium, a Roman mining technique described by Pliny the Elder consisting in the perforation of the mountain and latter irruption of great quantities of water that literally pulled the mountain down as you can see in the satellite image:
The romans removed 500 millons of m³ of land to get 3 grams of gold per tonne of land. Pliny stated that 20,000 Roman pounds of gold were extracted each year. The exploitation, involving 60,000 free workers, brought 5,000,000 Roman pounds (1,650,000 kg) in 250 years. To bring the necessary water from the Sierra de La Cabrera mountains to Las Médulas a system of channels more than hundred kilometers long was constructed, and some of its portions are still conserved. You can visit the channels with a guide and a helmet. The holes made by the water in the mountain are huge.
By the way, near las medulas you can visit the Castle from the Order of the Temple in Ponferrada.
Source: Las Médulas in Wikipedia