Under the watchful eye of M23 rebels in the hills around the Congolese town of Rubaya, a line of men in rubber boots ferry sacks full of crushed rocks up winding paths cut into the slopes.

The laborers are hauling coltan ore, a mineral that powers the modern world. The ore will be loaded onto motorbikes and eventually shipped thousands of kilometers away to Asia. There it’s processed into tantalum, a heat-resistant metal that fetches more than $300 a kilogram and is in high demand by makers of mobile phones, computers, aerospace components and gas turbines.

Rubaya produces around 15% of the world’s coltan, all dug manually by impoverished locals who earn a few dollars per day. Control of this mine is the biggest prize in a long-running conflict in this central African nation.

The area w

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