This is an issue in shaft mines but less in drift mines and slope mines. Do you really think that in Kivu, the miners have any type of pumping material?.
According to what you could see, they used drift mining at first and, then, after going several yards into the mountain, they switched to shaft mining.
No water pumps, no system to bring air, a high level of instability...
The temperature on the exterior is around 40°C and higher within the mine themselves. Miners can't stand up and there is only enough room for a single man. In fact, these mines looked exactly as the escape tunnels depicted in many movies.