Temperature increases through the Earth's crust average about 25°C per km over a 4 km depth, giving 100°C at 4 km on top of the surface temperature. However, geothermal heat flows over a continent are not uniform. The Mponeng Gold Mine (and the other deep gold mines in South Africa) are located over a "cold spot", where the increases in temperature with depth are only 9°C per km. Use this information to explain why it has been possible to mine such depths in this region.