There are over six thousands disused gold, chrome, diamond and platinum mines across South Africa that have created a thriving underground economy. One mining expert believes the illegal underground mining industry supports thousands of people above ground in small mining towns like Welkom where at least 34 "zama zamas" were killed in an underground explosion last week. Wits University anthropologist Prof. Robert Thornton, who specialises in studying illegal mining, says illegal mining has a very big impact on the economies of mining towns. Millions of rands in sex, food and alcohol is being traded in secret underground cities.