Those visiting art institutions expect a certain level of security - by that I mean signs that say 'Don't Touch' and gallery guards. It is an irritating but inevitable part of the gallery-going experience. We often behave in art institutions as we would in a church: in an atmosphere of deference, hushed whispers and silenced mobile phones. It is an atmosphere perpetuated by the institutions themselves and designed to increase the magnitude of the art object - this extraordinary, almost otherworldly act of expression that we are here witness. The Turbine Hall is a different kind of place, teeming with people, information points, telephones and an ATM. There is a vibrancy about the space that generates debate - it's the only place outside the classroom that I have found where my students are not inhibited in expressing their opinions. It is the kind of place where art needs to be, yet rarely is.
Public art is too often confined to places that are relatively inaccessible - on a deserted hilltop, a roundabout, a plinth, or submerged into the general hubbub of city life. The Turbine Hall is unique in being a public space designed specifically for art. Where else could Salcedo create such an installation? And where else could we have such freedom of access?
Enough silly-story carping about injuries: we should celebrate the rare opportunities we have to physically explore a work of art and rack up the sprained ankles to our lack of experience.
Det Sgt Helen Barnes on the look out for Borrower drug gangs
There has, obviously, been a great deal of speculation about the origins of the crack. The artist herself has let it be known that it took her a year to create and five weeks to install, and that bits of it were air-freighted across the Atlantic, but has refused absolutely to reveal her precise method. "What is important is the meaning of the piece; the making of it is not important," she says, adding that the work is "bottomless" and "as deep as humanity".
A spokeswoman for the Tate says firmly that it will never divulge how the piece - the eighth in its annual Unilever series of works commissioned specially for the Turbine Hall - was made. "The artist and Tate are not going into great detail other than to say we opened up the Turbine Hall floor in order to create a cavity," a spokeswoman says. "The work was made with utmost precision according to drawings by the artist, and nothing was accidental."
The press, for its part, reckons "concrete sections were lowered into a trench" (the Daily Telegraph), or that the artist "dug into a 'false' floor sitting on top of the original" (the Times). The Independent speaks of "realistic mouldings" and "visible fabrication".
No one, in short, has the slightest clue. Time to call in the experts.
Graham Merton, managing director of Eaton Gate, a prestige building firm operating at "the top end of the domestic refurbishment market", stands four-square across the fissure and rubs his chin. "What I reckon," he says, "is that they dug some of the old floor out - look, that slab there is definitely different, that's the original floor over there. It needn't have been much, maybe just 20 or 30 centimetres. Then replacement slabs were cast in a workshop somewhere, with the cracks already in them, and laid in situ. And where it gets deep down there, they could actually have dug down into the earth with a mechanical tool, and applied a hard slurry finish. No reason why not. But it's certainly impressive."
Ferhan Azman, an award-winning Turkish-born architect with lots of experience in concrete, kneels to probe the crack's sides. "Isn't it great?" she asks. "It works as art for me. It's about how our physical environment affects us. Look how wary, how destabilised you feel in a building with a great big crack down the middle. Anyway, it looks like they've taken a layer off the top here, and then in-filled with pre-cast pieces. It's not that mysterious. There'd be no problem digging down; with a building like this you could go on for ever without undermining its foundations."
Denis Ryan of TM Ryan & Sons casts the experienced eye of a south London builder over the work. "I'd say," he ventures, "that they've dug quite a narrow but quite a deep trench here, probably not much wider than the crack itself, then dropped in narrow pre-cast vertical slabs, all made off-site, to form the sides. Then you use a levelling compound to disguise the joins and make it look like you've replaced an entire slab of the floor. Whatever they've done, it's clever. They've got three builders here and none of us can really agree on the technique."
They can agree on one thing, though: they would all get sued for it. "This is extremely dangerous," says Merton, who otherwise likes the crack a lot, saying it reminds him of "an earthquake, like a reminder to look after the planet, to remember that everything, even the most massive structures, may be at risk. Art should do that, shouldn't it? Challenge you, make you think." Professionally, however, he warns you could "easily break a leg here. I'd never be allowed to let a building out like this. Heels will go in, ankles will get twisted, lawsuits will follow. Health and safety-wise, it's a disaster."
Ryan concurs, but jokes that if he tells his clients people are now paying good money to see eight-inch-wide cracks in the floor, "soon everyone will be wanting one". It probably is good art, though, he reckons: "It's got everyone talking about it, hasn't it? That's the main thing."
Azman only hopes health and safety do not get their hands on it. "There's been so much removal of commonsense from our lives," she says. "People may say children can fall in, but children could fall into the river outside. You just have to tell them to be careful!"
So I thank our experts and head out into the rain, considerably wiser about the techniques of concrete construction but, it has to be said, still not entirely sure I know just how that crack got there.
Mr E is a builder who was working at Tate Modern on another project while Shibboleth was being installed, and although for contractual reasons he does not wish to be further identified, he is very happy to recount what he witnessed. So here's the answer ...
"They dug a dirty great trench about a yard wide and a yard deep," says Mr E, still lost in wonderment. "Then they brought in lorry-load after lorry-load of cement and poured it in, using 10-foot sections of what looked like carved polystyrene moulding to form the sides. Then a whole bunch of people lay down on their stomachs for about a week and finished it off with brushes. Looked bloody uncomfortable, I can tell you. It's about racism?