From a distance the khaki rag looked like just another piece of rubbish blown against the barbed-wire border fence. Beneath it, on the ground, were a couple of sticks in the shape of an arrow pointing away from the South African/Zimbabwe frontier. Major Neels Smit, a South African border patrol officer, stopped his vehicle and walked over to the fence. Yanking at the rag, he discovered it had been carefully knotted to the wire. He peered at the lowest of the coils of wire that make up the fence. Then, with the gentlest of taps, he kicked open a "trapdoor". A minute later he had discovered 28 breaches in a 40-yard stretch. "The only way you can stop this is when troops deploy at arm's length apart," said a colleague. "They are coming like never before."
When, on 21 March, President Levy Mwana wasa of Zambia compared Zimbabwe to the Titanic and warned that people were "jumping out in a bid to save their lives", Harare reacted with outrage. Two nights later, state television beamed footage of empty roads around Beitbridge, the main border post, and denounced media reports of an exodus as western propaganda. The state cameraman must have worked hard - or never swivelled his lens to the right or left - to secure such unrepresentative footage.
Normally the border region does indeed have a sleepy feel, but now, as Zimbabwe descends into economic chaos, the area around Beitbridge has become the focus of probably the fastest-growing movement of people in sub-Saharan Africa.
Joseph Muleya works as a ranger on a game farm just ten miles south of the Limpopo River. For more than a decade, he has gone on pre-dawn patrols, but in the past year his game-viewings have suddenly become much more bizarre. Every day he almost literally stumbles on groups of Zimbabwean refugees. He has been shot at; he has had a bicycle stolen at gunpoint; ten days ago he came across a group of more than a hundred men, women and children; he has also come across scenes of utter misery. Late last year, he found the body of a Zimbabwean woman under a bush, with the body of a two-year-old child a few yards away.
"Last year, it was a question of finding one or two every now and then," he said. "Now I find them every night and they are many." Moments later, he jumped off his Jeep, shining a powerful game torch under a bush. There were Isaac and Nathan, eyes blinking in the light. The two young Zimbabweans in Gap T-shirts had crossed the border that same night. Clearly exhausted, they said they had not eaten for two days. "There are no jobs in Zimbabwe," said Isaac. "We used to work as electricians. Now we are unemployed. We were thinking of starting a business here." So were they supporters of Morgan Tsvangirai, the leader of the embattled opposition Movement for Democratic Change? "It is difficult to support him," Isaac said simply. "The police are everywhere."
The figures - and rate of increase - are extraordinary. Last year, the army arrested 72,000 people illegally crossing the border in the frontier zone. The year before, they caught about 48,000. The year before that they caught just over 26,000. And they reckon they capture only a tiny fraction of the people flooding south. South African officials estimate the number of Zimbabweans living in South Africa at three million. That seems on the high side, but what is not in doubt is that with inflation in Zimbabwe predicted to reach 5,000 per cent this year, the exodus is gathering pace.
For South Africa, this poses a huge challenge. While pursuing its policy of "quiet diplomacy" towards President Robert Mugabe's increasingly despotic regime, the government has been reluctant to speak out about the situation on the border. Indeed, many in the security services believe the government has sought to play down the influx in order to avoid having to admit that there is a crisis in Zimbabwe. This position, however, is becoming untenable. It is no longer just Musina, the nearest town to Beitbridge, that is swamped by Zimbabweans. Several of the townships and squatter camps around Johannesburg, 340 miles to the south, are also showing signs of strain - and xenophobia.
Nowhere is this more apparent than in Diepsloot, an informal settlement north-west of Johannesburg. Residents talk of Zimbabweans filling the clinics, of mounting tension between locals and newcomers. It is the fallout from this influx that appears to be stirring the government in Pretoria to become more involved in searching for a solution. President Mbeki's inner circle was this week given a dire assessment of the consequences for South Africa of a complete collapse of Zimbabwe. "It would be catastrophic for us," said one adviser.
All the while, the flood gathers pace. In Zimbabwe, there are villages that increasingly resemble the empty communities in Mexico whose young people have relocated across the Rio Grande. The South African army keeps the immigrants it man ages to catch in a small camp by the border before sending them back. This past week, a group of detainees were debating Zimbabwe's prospects and their own plans. "It is impossible to survive there," said Cleophas, who worked at a timber yard until he was laid off two years ago. Since then, his old wages have become meaningless when set against 1,700 per cent inflation. "I didn't have anything to give my children."
Another detainee, in a blue T-shirt, piped up: "You have to take a paper bag now to carry your wages." And a third, wearing an Arsenal strip, said: "He is very tough, Mugabe. You can't resist him. We've been patient. We thought something might happen. But now . . ." No one wanted to talk politics; most sat in silence. Then another load of immigrants arrived including several young women with babies. So what would they do when they were returned to Zimbabwe? They waited until the soldiers were out of earshot. Then they pointed to the river and mimed a border crossing. They will be back. And the chances are that next time they will make it.
Alec Russell is the southern Africa correspondent of the Financial Times