Mponeng is a gold mine in South Africa's Gauteng province. It extends over 4 kilometres (2.5 mi) below the surface, and is considered to be one of the most substantial gold mines in the world. It is also currently the world's deepest mine from ground level. The trip from the surface to the bottom of the mine takes over an hour.
Over 5400 metric tonnes of rock are excavated from Mponeng each day. At a price of $19.4 per gram of gold, the mine only needs to extract 10 grams of gold per ton excavated to remain profitable. The mine contains at least two gold reefs, with the deepest one metre thick.
The temperature of the rock reaches 66 °C (151 °F), and the mine pumps slurry ice underground to cool the tunnel air to below 30 °C (86 °F). A mixture of concrete, water, and rock is packed into excavated areas, which further acts as an insulator. Tunnel walls are secured by flexible shotcrete reinforced with steel fibers, which is further held in place by diamond-mesh netting.