'You see skeletons' - the deadly migrant Limpopo crossing

The men who raped Portie Murevesi did not care that she was visibly pregnant. They attacked her with glass bottles too, she told us, pointing to a large and jagged scar on her forearm.

Now, her pregnancy almost at full term, she is recovering at a church-run shelter in the South African border town of Musina.

"When I try to sleep sometimes, I see what those men did to me," she told the BBC. Musina is well known as a place of refuge for migrants who, like Murevesi, slip unnoticed
over the border.

The migrants who make it have survived a difficult trek through the bush. It is lawless and unforgiving territory. Wild animals and gangs of criminals are a constant threat.

Stories of theft, beatings, rapes and even killings are common.

"It is very, very dangerous," a Zimbabwean man, who only gave his name as George, told us.

"You see skeletons, you see someone already killed two or three months ago," he said about his own crossing.

We met him as night fell in Musina and groups of men returned to a dingy compound of tin-roofed huts.

The lucky ones had found some casual work in town, earning a little cash to send back to their families in neighbouring countries.

One explained: "We can't go back to Zimbabwe because there is nothing there. We're starving. There is no food."

No-one knows for sure how many undocumented migrants live, under the radar of the authorities, in South Africa, the continent's most advanced economy.

The last census found that there were more than 2,4 million foreigners - nearly half of them Zimbabwean - living in the country, accounting for just over 3% of the population.

But there are no official estimates for the number of those who have entered illegally.

And with a general election scheduled for the end of May, illegal immigration has become a highly charged political issue.

The South African authorities say they are tightening border security.

We saw for ourselves the enormity of the task.

Along the road from Musina towards the Limpopo River, which separates South Africa and Zimbabwe, coils of metal glint in the undergrowth.

It is the remains of a border fence: flimsy, piecemeal, trampled.

The river itself is all but dried up. And there, in the sweltering heat, dozens of people bustle back and forth across an invisible border.

Donkeys drag carts, laden with goods, across the cracked riverbed.

Women, balancing stacks of packages on their heads, hurry alongside.

They told us that it takes about five minutes to walk from the nearest Zimbabwean village into South Africa.

And there is nothing - no fence, no guards - to stop them.

John - who asked for his name to be changed to protect his identity - sat on his cart, occasionally twitching a whip at his restless donkeys. Watermelons were piled up on the wagon.

He has a family at home in Zimbabwe, he told us. But there are no jobs there, not enough food. So now he grows the melons and brings them over to sell in South Africa, where they fetch a far higher price.

"I do this to survive," he says.

It is a thriving, illicit, cross-border marketplace. While the migrants who cross here face a gruelling trek to Musina, most of the goods are shifted back and forth by cart or car.

But the South African government wants to take back control. Last year President Cyril Ramaphosa officially launched a new border force.

Mike Masiapato, the commissioner of South Africa's Border Management Authority (BMA), told us he was sending 400 newly trained officers to the border and procuring drones, body cams and motorbikes to improve surveillance.

"I can assure you now that the current leadership of the country understands the criticality of this work."

But even Masiapato acknowledges it will take time to really secure the country's border.

"We have started to fortify the environment. Hopefully in [the] next few years we should be able to succeed."

The country's governing party, the African National Congress (ANC), may not have years.

After three decades in power, the ANC presides over a country in which power and water supplies are failing and whose citizens are plagued by record levels of unemployment and violent crime.

As South Africa lurches towards what polls predict will be a bruising election for the ANC, it is perhaps unsurprising that some political opponents - like the anti-migrant party Operation Dudula - openly blame migrants for the country's woes.

Even Ramaphosa has said that undocumented foreign nationals exacerbate South Africa's social and economic problems

And other opposition parties are demanding stronger border controls, including ActionSA, which was formed four years ago by Herman Mashaba, an outspoken politician and former mayor of Johannesburg.

"The ANC government has failed our people terribly," says Malebo Kobe, a regional spokeswoman for ActionSA.

Kobe, who we meet by the Zimbabwe border, says that illegal immigration tops the list of voter concerns in this area.

She warns that local hospitals and other services have been overwhelmed by undocumented migrants who come here seeking healthcare or other benefits.

"It would be offensive to not even speak about the reality of what it does to our public systems when people don't pay taxes, but expect to live and benefit from the goods and services that our government provides."

Even as South Africa prepares - perhaps - to redraw its political map, need and desperation continue to define this country's limits.