It's the workplace of the "zama zamas," the name for the thousands of illegal miners who swarm the disused gold mines of Johannesburg every day.
Blessing Ndlovu demonstrates how he makes a living, chipping at the rock with a worn mallet and chisel, flakes dropping down into the blackness.
I suppress rising panic.
"When I came to South Africa, I never thought I would have to do this to eat," he says.
Ndlovu and his partner Respect Moyo have been gambling with their lives by climbing into the depths beneath the City of Gold. They use rudimentary tools and can spend days underground.
It is dangerous work.
"Sometimes the earth shakes and rocks like this fall down," says Moyo, lying prone and tapping his hand on a basketball-sized boulder. "When they become loose, they hit us."
Both have lost a friend to rock falls. But they say the real threat often come from rival zama zama gangs, who will kill miners deep under the earth for a sack of gold-bearing rocks. When they crawl out of the ground, they've been robbed at gunpoint more than once.
It's a brutal business. In 2014 a rival gang stole gold and trapped up to two hundred zama zamas by blocking the exits of an illegal mine. Some were saved by rescuers, but untold numbers died. It caused barely a ripple of outrage in South Africa, where xenophobia is deeply entrenched.
If the authorities catch them, the mostly foreign zama zamas face fines and arrest. But the police aren't making much headway.
"We try to catch them. But what can we do?" says a policeman who wants to know why we are filming the old mine rig.
I ask him if he would go down a mine to catch illegal miners.
"No, never. I have a young son. I'm not crazy," he says.
Mining companies see zama zamas as an expensive pest. The South African Chamber of Mines says that more than a hundred and fifty million dollars was lost to zama zamas between 1999 and 2004.
The government calls it a crisis and says illegal mining now brings in hundreds of millions in revenue each year. And as the gold price slides and more South African mines close their operations, illegal miners have rushed in as part of a sophisticated criminal network that smuggles the gold to international buyers, say experts.
But Ndlovu and Moyo are at the bottom end of the chain.
They fled the economic implosion of Zimbabwe and say their options as undocumented migrants are limited. With more than 50% youth unemployment in South Africa, migrants are often the last to find formal work.
They ply their trade in the disused shafts of Durban Deep mine. Once the poster child of the lucrative and deeply racist mining industry during apartheid, it closed more than fifteen years ago.
Now the old mining houses are being pulled apart by hand, brick by brick, to sell for construction. Faded signs still advertise the company golf course and Judo hall.
"Of course I don't like this job and I am scared all the time," says Moyo as he looks down the shafts, "but there is nowhere else I can find work."
Climbing up the shaft is a lot harder than going down. As we ascend, we see the faint glow of other miners' headlamps beneath us, flitting back and forth like fireflies.
As I climb out of the hole in the ground I think, yes, this has to be the worst job in the world.