The Mponeng mine in South Africa is the deepest in the world. Its basement operating depth is over 13,100 feet down* - which one reaches by descending a stagger of two obscenely long cage-elevator rides in total darkness, then hiking the last half-mile, into the furnace.
Because of the geothermal gradient, the rock temperature at the bottom of the mine exceeds 150°F (with 95% humidity). So they pour 6,000 tons of ice slurries down the mine's throat every day, to keep it just cool enough for people to dig and not die too often.
The guys who work down there are selected on the basis of how efficiently their bodies can thermoregulate. Most of them speak (or used to, anyway) something called Fanagalo - a pidgin of Zulu that serves as the lingua franca of subterranean Africa. That is, the language of hell.
There are more than 600 seismic events a day down there, rattling 236 miles of tunnels (that's 30 more miles than in the NYC subway system). And the miners are not alone in them.
Ore pirates dig their way down and penetrate the shafts, then take up residence underground - with wives, prostitutes, and other collaborators - going troglodyte for months at a time.
Their skin turns grey from lack of sun as they work remote corners of the mine abandoned by the operating concern with homemade explosives, DIY processing machines, etc. And occasionally the mine owner sends mercenaries down to hunt and kill them in the dark.
I don't really know what to say about all of this. To the extent that caves are like mountains turned inside out, it's an antiparticle of Challapalca in Peru. Every bit as terrible. And I can't stop thinking about it.
*To put that depth in context, the deepest natural cave identified so far, Veryovkina, is only 7,257 feet deep.