For their efforts, those toiling in these sought-after cooperatives can earn an average of 1,500 Bolivianos a week (£150/$225); those working alone usually earn a great deal less. It all depends on the quantity and quality of the minerals they produce.
Where precious silver was used mainly in coins and jewellery (and later in photography), its industrial uses now outstrip the decorative market. Silver has the highest electrical (and thermal) conductivity of any metal, so it is used in a range of electronics – including sensitive radio frequency antennae, particularly at VHF (very high frequency), such as found in televisions and mobile phones, and in radio frequency identification (RFID) devices. Silver is also found in many printed circuit boards, in hearing aids and in batteries.
The medicinal properties of silver bullets have been known since at least the times of Hippocrates, the Ancient Greek “father of medicine”, and rely on its toxic effects on pathogens, including bacteria and fungi. Silver ions kill pathogens by binding to proteins in their cells, making silver compounds ideal for use in antiseptics and wound dressings. Nanoparticles of silver are even woven into socks and other clothing to reduce bacterial and fungal growth – and the odours that arise. Silver is also used in heart valves and catheters, and researchers are now investigating silver's potential in killing cancer cells.
One solution to dwindling reserves is to recycle the silver already in circulation. A tonne of ore usually contains less than 3 grams (0.1 ounces) of silver; whereas a tonne of discarded mobile phones (6,000 handsets) can contain 3.5 kilograms (7.7 pounds) of silver, according to United Nations University experts.
Clearly, replacement materials are also needed. "As a surface coating, silver could be replaced by aluminium or rhodium, which is itself expensive, or by complex metal oxides, which are good conductors," suggests Peter Edwards, professor of inorganic chemistry at Oxford University. Other applications could use copper or silver-plated copper, or palladium alloys.
Finding replacements for medical applications might be more tricky. For instance copper also has an antiseptic quality – it is being used in hospital door handles to replace aluminium and reduce the spread of germs, and in water filters to prevent Legionnaires’ disease. But copper can be neurotoxic, so it is not used in implants or wound dressings, says Alan Lansdown, an expert in medicinal uses for silver at Imperial College London. "There aren't really any effective and safe replacements for silver, except synthetic antibiotics," he says.
So, with potential alternatives in short supply, and while the price is high and there is still some silver left in the ground, desperate people like those at Cerro Rico will continue to burrow for it.
After what feels like an entire day, but is in fact just an hour and a half, I emerge from Cerro Rico, blinking into the sunlight. Gasping the cold fresh air, I am grateful to be alive, but the dusty mine has taken my voice, I have to shower for 20 minutes to clean off the grime and my clothes stink of the underworld.
For me, it was a brief foray. I simply cannot imagine having to work in those conditions for 12-14 hours, sometimes doing a double night-shift, and for years. Children work in these mines from the age of 9 or 10. There are no middle-aged men, only widows shovelling rocks outside.