Hydraulic mining, which Pliny referred to as "ruina montium" ("ruin of the mountains"), allowed base and precious metals to be extracted on a proto-industrial scale. The total annual iron output is estimated at 82,500 tonnes. Copper was produced at an annual rate of 15,000 t, and lead at 80,000 t, both production levels unmatched until the Industrial Revolution; Spain alone had a 40 percent share in world lead production. The high lead output was a by-product of extensive silver mining which reached 200 t per annum. At its peak around the mid-2nd century AD, the Roman silver stock is estimated at 10,000 t, five to ten times larger than the combined silver mass of medieval Europe and the Caliphate around 800 AD. As an indication of the scale of Roman metal production, lead pollution in the Greenland ice sheet quadrupled over its prehistoric levels during the Imperial era, and dropped again thereafter.