GOMA - After two hours, drenched in sweat, he tugs on a cord tied to his waist and is pulled back to the surface, carrying with him a 30 kilogram sack of raw columbium-tantalite ore.
Few people have heard of this rare mineral, known as coltan, even though millions of people in the developed world rely on it. But global demand for the mineral, and a handful of other materials used in everything from cellphones to soup tins, is keeping the armies of Congo's ceaseless wars fighting.
More than 80 per cent of the world's coltan is in Africa, and 80 percent of that lies in territory controlled by Congo's various ragtag rebel groups, armed militia and its corrupt and underfunded national army.
Despite Friday's ceasefire summit in the Kenyan capital Nairobi, and visits to Congo by earnest international politicians and diplomats, there will be no peace until the economic forces driving the conflict are addressed, experts warn.
"Until now, this question has been avoided on the basis that it is too sensitive or could derail peace talks," said Patrick Alley, director of Global Witness, a British charity which has investigated the militarisation of Congo's mineral trade.
"That is a short-sighted view. If international dialogues continue to ignore this critical aspect of the conflict, they will not find long-term solutions."
In Congo's North Kivu province, scene of the current bloody conflict, the supply chain that links the sweating miner to the mobile telephone in your pocket starts around Masisi district, the rebel-held area 110 miles northeast of the provincial capital, Goma.
Back up on the surface again, the miner hands his sack of ore to his shift boss, who pays him less than a dollar per kilogramme. Some mines also use child labour, often for no pay at all.
The rocks are then packed into even heavier 50kg loads and passed to porters, who hoist them on to their backs and set off, in flip flops or Wellington boots, for the two-day walk through the mountains to the town of Walikale.
There, the ore is sold once again, now for just over a dollar a kilogramme, to a middleman known as a negociant. He consolidates several loads and calls in an aircraft to land at the town's grass airstrip, collect the rocks and fly them to Goma.
Dotted across Goma, behind high walls and locked gates, there are hundreds of small-scale traders called comptoirs. Men in dusty overalls sit with large piles of rocks in front of them, using a trained eye to scan scan for the chunks likely to yield the best-quality product, samples of which they then grind to assess its coltan purity and how much to pay the negociant accordingly. In an office to the rear, the comptoir director sits in front of his laptop, scanning coltan and cassiterite prices on the internet site of the London Metal Exchange.
"Things have progressed a bit today because we are able to see what is the best price instantly, rather than having to guess as we did before the internet," said Joseph Nzanzu, a comptoir director in Goma.
"But still the process, the negociants, how they come to us with the ore, how we grade it and argue over the price, this is the way it has been for decades."
Gathering hundreds of kilogrammes together, the comptoir loads the ore on to trucks which set off for Mombasa on Kenya's Indian Ocean coast, five days' hard driving away through Rwanda, Uganda and Kenya.
From here, cargo ships carry the coltan to processing plants in the Far East, although it is also traded as a commodity on the London Metal Exchange and in Belgium, Congo's former colonial power. The ore, still hunks of rock just as it was when it came out of the mine, is ground down and refined to extract tantalum, a heat resistant powder which is sold to firms making the capacitors which are found in mobile telephones and other electrical devices.
Finally, the equipment manufacturers buy the capacitors, without which their goods would not work. From North and South Kivu, a total of 428 metric tonnes of coltan was exported in 2007, according to the provincial ministry of mines, worth around £2 million. But these figures are notoriously inaccurate, and take no account of illegally smuggled minerals, likely to make up almost as much again.
There is nothing illegal in buying or using coltan, despite concerns that some of profits from the trade in the Congo helps fund its myriad armed groups. All of the big electronics manufacturers say that they make every effort to ensure that the components in their products are from legitimate mines, either in Congo or in other coltan-producing countries including Brazil and Argentina.
But in Congo's anarchic environment, it is impossible for customers to know for sure that the tantalum in their mobile phone, DVD player, PlayStation or desktop computer did not come from a rebel-held mine. Buyers say that ore from these mines is mixed with that from legitimate mines, and they cannot tell which is which. There is no equivalent of the Kimberley Process, the international system which certifies that diamonds are from conflict-free areas.
The links between Congo's vast riches and its blood-stained history stretch back to the Belgian colonial era, when King Leopold II forced labourers onto his rubber plantations and ordered his agents to chop off the hands of workers who failed to fulfil their harvest quotas.
But throughout the latter half of the 1990s and the beginning of this decade, as Congo descended into two wars, its mineral wealth began directly to stoke its conflict. At the height of a coltan price boom in 2001, the UN estimated that rebel groups were earning $20 million a month from mineral exploitation, though the market price has since fallen.
A 2003 United Nations investigation into the illegal exploitation of natural resources accused both Rwanda and Uganda of prolonging their armed incursions into Congo in order to continue their plunder. Peace was supposed to have come to the region that year. But in the east, the rebels and armed militia remained and proliferated, extending their reach into the mines opened by a series of state mining companies and then abandoned as war swept the country.
Today, these armed groups earn their money either by directly controlling the mines themselves, or by taxing lorries as they pass through their territories. Alongside them, Congo's own army runs various mines and its officers pocket the profits.
There have been calls for an international embargo of the trade in the country's minerals. But that would only hurt its poorest citizens, who have little else to do to earn money, said Mr Nzanzu.
Instead, according to Mr Alley of Global Witness, buyers must double efforts to ensure that they do not trade in any mineral tainted by contact with any of Congo's armed groups.
"For as long as there are buyers who are willing to trade, directly or indirectly, with groups responsible for grave human rights abuses, there is no incentive for these groups to lay down their arms," he said.
"It is not acceptable for buyers to claim they do not or cannot know where the minerals come from. They have a responsibility to find out exactly where the minerals were produced and by whom.
"If there is any likelihood that they have passed through the hands of armed groups or army units, they should refuse to buy them."