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; Hispania alone had a 40% 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. Transportation and communication The Roman Empire completely encircled the Mediterranean, which they called "our sea" (mare nostrum). Roman sailing vessels navigated the Mediterranean as well as the major rivers of the Empire, including the Guadalquivir, Ebro, Rhône, Rhine, Tiber and Nile. Transport by water was preferred where possible, and moving commodities by land was more difficult. Vehicles, wheels, and ships indicate the existence of a great number of skilled woodworkers. Land transport utilized the advanced system of Roman roads, which were called "viae". These roads were primarily built for military purposes, but also served commercial ends. The in-kind taxes paid by communities included the provision of personnel, animals, or vehicles for the cursus publicus, the state mail and transport service established by Augustus. Relay stations were located along the roads every seven to twelve Roman miles, and tended to grow into villages or trading posts. A mansio (plural mansiones) was a privately run service station franchised by the imperial bureaucracy for the cursus publicus. The support staff at such a facility included muleteers, secretaries, blacksmiths, cartwrights, a veterinarian, and a few military police and couriers. The distance between mansiones was determined by how far a wagon could travel in a day. Mules were the animal most often used for pulling carts, travelling about 4 mph. As an example of the pace of communication, it took a messenger a minimum of nine days to travel to Rome from Mainz in the province of Germania Superior, even on a matter of urgency.