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Illustration: Bryan Christie
The Mponeng gold mine in South Africa is the world’s deepest mine, extending more than 2 miles below the surface. Not surprisingly, conditions at its depths are hellish. The commute down takes more than an hour. The rock itself can reach 140 degrees Fahrenheit and occasionally explodes. But there’s also a pair of gold reefs down there, the lowest of which is 3 feet thick. It costs about $550 an ounce to extract the bling—not too bad when gold is selling for $1,300 an ounce. AngloGold Ashanti, which runs Mponeng, can keep well in the black by recovering just 0.35 ounce of gold from each ton of rock, and Mponeng pulls up 6,000 tons a day. AngloGold and its chief rival, GoldFields, dug the first ultradeep mines decades ago, but only recently has technology made the operations (sort of) safe. Here’s how they do it.
Steel ropes
Shaft elevators, called cages, are three-level giants that can hold up to 120 miners, and the lines they’re suspended from may be up to 10,000 feet long. Many mines use eight 2-inch-thick triangular-strand steel ropes encased in plastic, each with a carrying capacity of 20 tons.
Seismic monitors
Mining at 2 miles below the surface has a way of … disturbing things. Ultradeep mines use seismic monitoring stations—7-inch cylinders packed with sensors in 30-foot- deep boreholes—to transmit signs of movement on the scale of inches to mine managers.
Shotcrete
The tunnels leading to the mining area bulge and swell and are prone to sudden rock-bursts. So crews spray the walls with shotcrete, a flexible mixture of concrete and steel fibers. Diamond-mesh netting holds the shotcrete in place but allows the rock to creep slightly.
Refrigeration
Mponeng relies on systems that pump ice slurry underground to cool tunnel air to less than 85 degrees. (Excavated areas are packed with a mix of concrete, water, and rock in massive polypropylene socks. The concrete hardens, leaving a support that also acts as a gigantic thermal barrier.)
Jumbo drills
Air-powered hand-held drills work the small tunnels; for large shafts, miners deploy monsters like the Atlas Copco 282, a two-boom jumbo drilling rig capable of working three times faster than air drills alone. With an Atlas, miners can blast away 10 feet of rock per day instead of just 3.
Ventilation
Air doesn’t flow all that well 2 miles underground, so you need giant fans—some as much as 14 feet wide—installed on the surface atop ventilation shafts. Variable-pitch fan blades that can be adjusted in mid-suck (or blow) enable the airflow to be quickly and easily regulated.
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