They're called the Zama Zama - the men who barely make a living searching for missed fragments of gold in South Africa's long-disused gold mines.
It's a desperate, dangerous way to live. So what drives Zimbabweans to uproot in search of the illegal work?
ITV News Africa correspondent John Ray reports:
"There’s something I want you to know about me," Wellington Mpofu says, almost apologetically.
"I have my education. My primary and my secondary. I passed. But in Zimbabwe, the economy is going down. There’s no work."
We’re speaking above a great chasm in the ground. Far below, the small, rocky entrance to a mine - long abandoned to everyone but men like Wellington.
He’s agreed to take us down. But first he wants to explain what's brought him to South Africa.
"Everything is money, money. I have to pay my rent. And my children are in school, and I have to pay their fees."
His wife and family are back home in Zimbabwe, where once - in another life it must seem - Wellington was an opposition activist working against Robert Mugabe.
But Zimbabwe's president stayed in power and Wellington became one of the many thousands who have made their way to Johannesburg, not to seek their fortune, but to eke out a hazardous living.
They’re called the Zama Zama. From Zulu, a rough translation would be 'men who try to get something from nothing'.
Today, Wellington is heading down the mine. He has a torch, a hammer. Usually he carries enough food for two weeks.
He’ll spent that long, lonely time searching the miles of disused mines that run underneath the city for rocks that he hopes contain tiny fragments of an elusive precious metal.
Johannesburg was built, literally, on gold, but the greatest seams are exhausted.
Wellington and his kind are searching for crumbs from a lavish banquet long finished.
We step just a few metres into his subterranean world. Quickly the darkness engulfs you. The jagged ceiling is low, the air cool and suddenly damp. I soon long for the surface and sunshine.
"I don’t like to come down here. It’s dangerous. Very dangerous," says Wellington. "My wife prays for me when I tell her I am going to work."
He talks of the constant dangers of rock falls, of toxic gases.
No one knows who many Zama Zama miners are killed each year.
Conservative estimates start at 20 - but no one is counting. And often the victims of underground accidents are never recovered.
Ready to head deeper into the pitch black beyond, Wellington turns to me.
"No one would choose to do this," he says. "But what else is there for me?"