The world's largest opencast mine is more than 4km/2.5mi in diameter and nearly 1 200m/3 940ft deep. It is one of only two man-made objects that can be seen from outer space, the other being the Great Wall of China. Since mining operations began in 1906, 6 billion tons of earth have been extracted in order to extract copper ore, molybdenum, silver and gold with a value greater than the Californian gold rush, Comstock Lode and Klondike combined.