What is the deepest mine on Earth?

Mponeng, a gold mine southwest of Johannesburg, working nearly four kilometres below the surface. The virgin rock temperature down there is around 60 degrees Celsius, so the mine runs an air-conditioning operation using thousands of tonnes of slurry ice pumped underground each day just to make the workings survivable.

The journey to the face takes over an hour in stages, in cages carrying more than a hundred people at a time and dropping at around fifteen metres a second. The rock is under enormous stress at that depth and fails explosively - a rockburst, effectively a small earthquake caused by mining - which is the main hazard and the reason South African deep-level gold mining has one of the worst safety histories in industry.

It also produced a biological surprise. Water from fractures kilometres down turned out to contain a bacterium, Desulforudis audaxviator, living entirely alone in an ecosystem of one - drawing its energy from the radioactive decay of the surrounding rock rather than from sunlight at any remove. It is the standard example of what life might look like beneath the surface of somewhere with no sun at all.