The numbers behind gold and gold mining are as fascinating as the metal itself. As an example, the U.S. Geological Survey estimates the total amount of gold minedworldwide through 2015 at 187,000 tonnes (tonne=metric ton=32,151 troy ounces), or roughly 6 billion troy ounces. Because gold is so dense and the magnitude of its numbers is so great, the volume of this cumulative production is di_ cult to visualize. But 6 billion troy ounces of gold would occupy a cube measuring 69.2 feet on a side. Expressed di_ erently, it would cover a regulation NBA basketball court to a height of 69 feet, or _ ll 61 standard railroad boxcars. Because it resists oxidation and corrosion, and because we value it so highly, hoard it, store it in secure repositories, and recycle it, 98% of all that gold can still be accounted for today.

Central banks store 20% of it, private investors hold 19%, 47% is fashioned into jewelry, and 12% is fabricated into industrial, scientic and decorative forms. Only about 2%, or 120 million troy ounces, has been lost to dissipative industrial uses or is otherwise unrecoverable.

The historic rate of gold production has varied radically. Despite such celebrated events as the California, Australia and Yukon gold rushes of the 1800s, only 13% of the cumulative gold production was mined prior to 1900. In fact, half of it was mined in just the last 45 years, a period that represents the greatest gold mining boom in history.

And that boom has not yet reached its peak. In 2015, annual global production set another new record—3,000 tonnes, or 97 million troy ounces. But while production is at a record high, the average grade of the ores being mined today has fallen to a record low—a mere 0.03 troy ounce of gold per ton.

During the 19th century, gold ores graded at least 1 troy ounce per ton, but by 1900, these high-grade ores were largely depleted. Most remaining gold deposits were too low grade to be mined and processed pro­fitably using traditional technologies. But during the following decades, a series of technological advancements and economic events set the stage for today’s gold mining boom.

The early 1900s saw the introduction of mechanized mining practices, flotation separation milling, and cyanidation extraction processes. ‑ These advances, which made it possible to mine lower-grade ores, sustained the gold mining industry for the next half century.

But by 1950, faced with continuously declining ore grades and increasing operating costs, gold mining was again in trouble. The biggest concern was the price of gold itself which, since 1934, had been ­ fixed by governmental decree at $35 per troy ounce.

Nevertheless, gold mining was about to make a remarkable recovery, and the ­ first step was to improving the cyanidation process. Although miners had used cyanide to extract gold from ores since the 1890s, recovering the dissolved metal from solution was ineffi cient and costly. That problem was solved in the late 1950s, when U.S. Bureau of Mines researchers developed the carbonin-pulp (CIP) recovery process, which uses activated charcoal to adsorp dissolved gold from cyanide solutions. e fast, inexpensive CIP process ­finally enabled cyanidation to reach its full potential in recovering gold from low-grade ores.

In 1967, Nevada’s Carlin mine, an open-pit operation in which ore graded only 0.1 troy ounce per ton, demonstrated on a commercial scale that cyanidation-CIP extraction recovery represented the future of gold mining. The Carlin mine became the model for hundreds of modern gold-mining operations worldwide.

But even with mechanized mass mining and cyanidation-CIP, the gold mining industry still needed an economic boost that came in 1974, when governments abandoned their efforts to hold the price on gold and turned the metal loose as a free-market commodity. By the early 1980s, a er gold prices had settled into the $300- to $400-pertroy-ounce range, the modern gold mining boom was underway.