The tunnel grew darker and more claustrophobic, the air harder to breathe. But Daniel Cruz trudged deeper into the bowels of a mountain where thousands of miners toil and countless more are entombed, casualties of a centuries-old lust for silver.

Behind him followed five foreign tourists, here to see an anachronism in the 21st century, medieval mining in the Rich Mountain of Potosí. This cone-shaped peak is at any given moment home to as many as 16,000 shirtless miners, straining in dark caverns with picks, shovels, their own brute strength but little else.

"They have to choose money or life," said Cruz, who is 27; he started mining at 17. "When they choose to mine, they are risking their lives."

This relic from the days of colonial Spain brings young tourists such as Charles Newman, who on a recent day watched Indian Quechua miners endure 40C heat below the earth.

"I thought the working conditions were pretty shocking, coming from Europe," said Newman, a 19-year-old Briton. "It's quite humbling, actually seeing what they do on a daily basis."

Tourists, though, may not be able to travel into the Rich Mountain for long, at least if the engineers who have been studying it are right. They say that after 467 years of mining, the Rich Mountain is like Swiss cheese, pockmarked throughout and in danger of a catastrophic collapse.

The mountain is still as imposing as it was when Spanish soldiers, their armour clanking, first set eyes upon its red, rocky hues rising 4,800 metres into the deep blue skies of Bolivia's southern highlands.

Nestor Rene Espinoza, a veteran engineer who recently completed the most extensive study of Rich Mountain's stability, said the evidence of volatility is hard to ignore.

There are 600 mines, he said, and 100km of tunnels. Gaping, unmarked shafts drop to oblivion. Runoff turns soil to mush. Part of the mountain's iconic cone has already collapsed into itself.

"We've taken all manner of riches from the mountain, but we have not given it the love it now needs," Espinoza told representatives from Potosí's mining sector on a recent night. "I've said it before, the mountain is ill, and it requires medical care."

Espinoza's study, unveiled in August, showed that five large areas of the mountain's upper levels have a high risk of collapse. The best solution, he said, is to pump cement into the abandoned shafts, an operation that would cost at least $300m and take more than a year.

That idea is not going down well in Potosí, where the Rich Mountain employs one in every nine inhabitants.

It all began in 1545, when an Indian shepherd, who was forced to spend the night on the mountain, started a fire and "saw a white and shining vein – pure silver", Uruguayan author Eduardo Galeano wrote in the most popular account of the mountain's history.

The Spanish created a forced-labour system, the mita, putting to work an estimated 33 million Indians over the next 250 years. And what they produced provided a steady stream of riches to Spain.

With dozens of deaths taking place each year inside the mountain, miners long ago came up with a brutally fitting moniker for their workplace: The mountain that eats men.

"What's happening inside the mine is medieval," Serrano said.

The mines have no lighting, no safety regulations or inspectors, no modern rail cars and no pumped-in oxygen, leaving miners to inhale a fine deadly dust.

"With these lungs, at night I can't even sleep anymore," said Elogio Tola, who started at 16 and is now 45, old for this mountain. "When you start to cough now, you cannot stand it like when you're young."

Those who work here – including hundreds of boys, some as young as 14 – are under no illusions.

Chewing wads of coca to ward off hunger and exhaustion, some heft huge bags of rocks to the surface on their backs. Smaller, nimbler miners specialise in burrowing into tiny crevices to place dynamite charges. Most spend the workday like their ancestors did: breaking through rock with hammer and iron chisel.

For their troubles, most earn a paltry $14 a day, even as a commodities boom is generating far more earnings than in the past.

"For us, there's nothing," said Wilber Marino, 41, sweat covering his bare chest, as he took a break in one dark cavern to talk about his life. "It's for the boss. We work like mules. We work here like slaves."

The paradox is that since the 1980s, when the state-run mine laid off thousands, the mining has been carried out by co-operatives controlled by the miners themselves.

But the 35 co-operatives that now mine mostly lead and zinc are unregulated, pay no taxes and critics say they exploit those miners who aren't lucky or shrewd enough to have become co-op bigwigs.

"They're called co-operatives, but they're not really co-operatives," said Celestino Condori, head of a local group of activists, the Potosí civic committee. "They are companies with peons."

But the miners say they'll never abandon Rich Mountain, no matter how unstable it becomes.

"We have to continue to work here," said Zenon Guzman, 33, who began here at 12. "Where are so many people going to go?"

• This story appeared in Guardian Weekly, which incorporates material from the Washington Post