Ponte towers by day, but at night it really comes into its own: the huge Coca-Cola sign mounted on its top floor is visible for miles, blinking against the darkness. The building is more than simply a landmark, though: its history reflects social change in Johannesburg, and its possible future is a metaphor for turbulent times.
Ponte opened in 1972 as a sumptuous "whites-only" apartment block, each with a breathtaking view. But its concrete and chrome were exhausted long ago. As the area slipped down the social scale, the building became the first port of call for immigrants from countries such as Greece and Portugal, brought in to swell white numbers. As apartheid began to founder, moreover, the neighbourhood around Ponte was one of the first to be unofficially desegregated. Now the circular tower, like its surroundings, has become almost entirely black; the whites have fled to distant suburbs. Grime dulls its windows and, inside, its corridors are shabby.
What is one to make, then, of Ponte's latest reinvention? Once the highest residential building in sub-Saharan Africa, it may now become the world's tallest jail. The South African government, struggling with a crime wave and a chronic shortage of prison accommodation, is seriously considering housing thousands of prisoners in the tower in teeming, already crime- ridden Hillbrow.
At first Henning Rasmuss, 30, a local architect, thought it was a joke. "There have been so many myths and stories about Ponte," he says. "It was rumoured the Nigerian government was going to buy it, because it was already full of its citizens, and also that the ANC was going to fill it with returning exiles." Now locals joke that Ponte is so full of illegal immigrants that all the government has to do is put up the bars; the prisoners are already in there.
That Ponte Prison is actually a possibility horrifies Mr Rasmuss. What does it say when the building which dominates a city, already internationally infamous for crime, is turned into a jail? "At a symbolic level it sucks," he says. Inner-city Johannesburg, he argues, is dying not so much from crime and grime as the "negative perceptions" of whites, bitter at their loss of ownership. Ponte Prison will hardly encourage deserters back or new residents to stay, though he does not expect that to be a consideration when the low-cost scheme is presented to a beleaguered government.
Paul Silver, the American architect behind the proposal, says Ponte is ideal as a jail. South Africa has an embarrassingly high level of prisoner escapes, most often during transfers between police cells, courts and jails. Ponte is within a few miles of the existing city courts and is even being promoted as a one-stop "justice centre" incorporating court facilities as well as cells, "eliminating the need to move inmates at all".
Does Mr Silver worry about the symbolic message? "It is actually a nice way of saying we are doing something about crime," he insists. "You don't hide penicillin just because it reveals that there is disease." Crime levels around prisons actually fall, according to the architect, and Hillbrow could do with that. In any case, the concrete tube is so ugly residents will not miss it.
But the truth is that while fleeing whites sniffed at Ponte, those who live there now love it. Last week Charlotte and her husband, behind with rent, tied two stained mattresses onto a bakkie (pick-up truck) and headed back to Soweto. "I love it here," said Charlotte. "But my husband is not working."
Students are in fact Ponte's largest tenant group. For 350 rand (pounds 42) each a month the luckiest share the top-floor penthouses. They may have lost their original glamour but still come with working sauna, en suite bathrooms and built-in bars - for drinks, that is, not on the windows. They are appalled at the prison proposal, as are the tower's immigrants from Africa's poverty-ridden trouble spots. For them Ponte is a symbol of what might be possible in a new land. Their protests are muted, however, no doubt because many lack the necessary residence papers.
"The Ponte needs upgrading," admits Sipho, a 34-year-old South African resident. "But it should be for ordinary people, not criminals." He loves the Africanised city centre: "When I was a little boy Ponte was just for the elite, and blacks who came to Johannesburg did their business quickly and got out. We were not welcome."
For a surprising number Ponte is still the stuff of dreams. Ihlaan Hendricks, 22 and coloured (mixed-race), has just arrived from Cape Town. Although alone and a little lost in Johannesburg, she thinks her tiny and bare room in Ponte is heaven.
"My mother always said never go to Hillbrow. But people here are nice," she said. "I saw this building the day I arrived, but I never thought I would live here. It is wonderful to stand at the window and look out over the city at night. Everything twinkles like a fairy tale."
For Ponte, though, there may be no fairy tale ending. When the government considers prison bids next month it is rumoured Ponte's owners - who say they are losing money - will be as desperate to off-load the building as the government is for cells.