So the Spanish adopted the mita, which the Incas had used to build roads and public buildings, to mine and process that silver. So one-seventh of the adult male Indian population from each district was required to work in the silver mines for a year, being paid only subsistence wages. Now, you might wonder why the Spanish didn't purchase African slaves to work in the mines. They did in Mexico, but in South America, it was cheaper to use indigenous labor. Purchasing slaves was inefficient because, one, they didn't have experience working at high altitudes, and two, mine work was super deadly. Mercury, which can be used to refine silver ore, was found at the mountain of Juan Cavalica. And mercury poisoning among miners was so common that parents would often maim their children to keep them from having to work in the mines.