On 29 July 1999 a team of AngloGold mineworkers at the Mponeng mine in Carletonville accidentally drilled into a pocket of flammable methane gas in the rockface. They had been lengthening a tunnel to a working area when the explosion took place. The lengthening process requires the drilling of long holes into unmined rock to check for pockets of water or gas and the workers had been following the correct procedures before the accident. As a result of the immense pressure, the gas it rapidly spread throughout large parts of the mine. The group sounded the alert when they detected the methane and evacuation started immediately. The gas was ignited during the process of evacuation and 18 miners died, with the death toll rising to 19 in the following days.

Carletonville is situated about 100km from Johannesburg in the Gauteng Province.
(Source: http://www.routes.co.za/maps/gp/carletonville/carletonville.jpg)

Mponeng Mine saw the death of 18 miners in a gas explosion.
(Source:http://www.explosives.co.za/images/blastingnews/sept2001/mponengmine.jpg)

20 highly trained rescue workers lifted 20 miners from the mine immediately after the accident and they were transported to hospital from where they were all given a clean bill of health. The rescuers found 17 bodies in the 2 700 meters shaft and continued to search in poorly ventilated tunnels filled with poisonous clouds of smoke and gas until they found the body of the 18th man. Another corpse was found later, bringing the total death toll to 19. The explosion did not disrupt mining, which continued the next day.

An inquiry went ahead and it was found that AngloGold was not responsible for the deaths of the miners and could not be held liable.

Sources:

http://www.miningweekly.co.za/min/features/health/?show=49794
http://www.cnn.com/WORLD/africa/9907/30/s.africa.explosion/