So, I'm standing astride this 548ft crack that that has rather alarmingly appeared in the floor of Tate Modern. I'm with an architect and a couple of builders, and we are examining the crack from a wide variety of angles and sticking our fingers inside and giving it a damn good poke and generally trying very hard indeed to work a few things out. The first is: how on earth did it get here? The second is: could it be dangerous? This being the Tate, we also feel obliged, finally, to consider the possibility that it might be art.

We are each clutching a black leaflet that reads: "Warning. Please watch your step in the Turbine Hall. Please keep your children under supervision." The leaflet explains that the crack is by a Colombian artist, Doris Salcedo, and that it is called Shibboleth, which, according to the Oxford English Dictionary, is "a word used as a test for detecting people from another district or country by their pronunciation; a word or a sound very difficult for foreigners to pronounce correctly". The Old Testament relates that the Ephraimites were trying to cross the river Jordan when they were caught by their sworn enemies the Gileadites, who forced them all to say the word "shibboleth". Since the unfortunate Ephraimites' dialect did not include the sound "sh", this allowed the Gileadites to identify and slaughter large numbers of Ephraimites. So a shibboleth, the leaflet continues, is "a token of power: the power to judge and kill".

None of this, though, is of much immediate concern to us, because what we want to know is how Salcedo got the crack to appear in the first place. This is, after all, as solid a piece of postwar real estate as you are likely to encounter; a massive former power station built in 1947 and comprehensively renovated in 1981. And the crack is big. It starts as a tiny hairline fissure in the concrete floor, beneath a wastepaper bin (is this significant? Reader, we confess: we do not know) up at the hall's western entrance, gradually widening and deepening as it descends in a crazy zigzag, with branches shooting off here and there, until it disappears under the wall at the other end. At its widest it is maybe 10 inches across; at its deepest it is maybe a three-foot crevasse.

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. Fortunately, at the end of the phone is Mr E.

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? Can't see it myself, but I'm not much of a one for modern art. It was a pretty good trench, though. And one hell of a lot of cement. Good luck to 'em."