The discovery of gold on the Witwatersrand in 1886 quickly turned Johannesburg into a boom town. Today, mine dumps comprised of crushed rock are scattered across the city. There are about 200 dumps in and around Johannesburg, some up to 50 metres tall and containing about 438 million tons of mine dump soil.
Early gold extraction processes were not very sophisticated, so there are still traces of gold in the dumps. In some cases, these are now being extracted using modern techniques. A dump is considered viable when 0.4 grams of gold can be obtained from every ton.
To minimise the dust blowing off the sand dumps, grass and shrubs have been planted on the surfaces. The dumps are, however, a major source of air pollution on windy days.