One of South Africa's biggest gold firms has taken the drastic step of banning all food underground to cut supply lines to gangs of illegal miners used to staying deep in the mines for months on end, threatening lives and official production.
With gold mining around Welkom, 200 km (130 miles) south of Johannesburg, dating back to the 1930s, the bedrock is criss-crossed by a myriad network of tunnels that provide perfect cover and multiple entry points for illegal miners.
Bosses of Harmony Gold's 2.4 km deep Phakisa mine - one of the world's deepest - have tried blocking up old shafts and installing stadium-style turnstiles at the top of the main shaft to stop imposters slipping through.
In January this year, they tightened the screw by imposing a total ban on food to prevent official miners bringing in supplies to sell or give to their unofficial counterparts.
There are two things you need to survive underground: food and water. You can always get water down a mine but the food ban has made a real difference, Harmony chief executive Graham Briggs told Reuters this week during a mine visit.
Unions agreed to the ban - as long as it was accompanied by a free meal at the end of a shift - even though it means teams of men will consume nothing but water during an eight-hour shift pounding at the gold-bearing rock in sweltering heat.
Although nobody knows the full extent of a problem that is literally hidden deep in the bowels of the earth, the countermeasures introduced by firms such as Harmony suggest the threat from illegal mining in South Africa is significant.
Once underground, the men will stay there for weeks, if not months, subsisting on food brought in from above ground.
They make a living by crushing the ore by hand and panning out the specks of gold or lighting fires beneath ad hoc smelters. In some cases they will even undertake their own drilling and blasting - at great risk to themselves and others.
Reports of underground clashes between armed gangs and mine security officers are common in the domestic media, as are accidents caused by unofficial mining activity.
In March, up to 20 men were killed after a rock fall at an abandoned gold mine near Johannesburg, and at least 10 died in May when a tunnel collapsed at a disused diamond mine.
Although mine owners are not blamed for such tragedies - and will often send in their own rescue experts to pull out victims - an aggressive zero harm government safety push means they cannot afford to have outsiders wandering around underground.
There's a very big risk to safety in these mines because illegal miners could mine pillars, boundary walls and basically dismantle the structures established to ensure stability, said May Hermanus, a former chief mines inspector.
That's very serious.
Despite the success of Harmony's food ban, the overall threat is unlikely to go away while gold is fetching $1,600 an ounce and South Africa's 25 percent unemployment rate pushes many young men to risk everything to eke out a living.
These guys are crazy, Briggs said. They will try and go down a vertical shaft with just a few bits of old rope.