It has become an instant popular success. The latest in Tate Modern's Unilever Series of installations in what was the turbine hall of the former power station draws the crowds in numbers not seen since they swarmed to the warm glow of the huge sun of Olafur Eliasson's "Weather project" in 2003-4. I'm not counting those helter-skelters they had a while back. They were fun, but they weren't art. Sorry.

Doris Salcedo's "Shibboleth" is a crack snaking across the concrete floor of the hall, getting wide and deep enough in places to twist your ankle in. People stare down it, put their hands in it, wonder aloud how it was made. Then get out their phones and take photos of each other straddling the great divide.

So I got out my phone and took similar pictures, and here they are. There's no doubt that it's intriguing. Not so much how they made the crack, but how they lined it, narrow as it is, with the cement-and-mesh finish that Salcedo favours. You can see evidence of patching of the concrete screed to either side of it, but one is left wondering what dentist-like tools must have been employed to create the effect.

Its effect is remarkable. School art classes turn up, sit down beside it, and sketch it. That's sketching a crack in the floor. People walk crabwise, bent double, as if expecting to find some evidence of life, a portal to the centre of the earth, bubbling water - who knows?

The best view is from the gallery above, where you can see how the crack attracts people in knots to its mysterious junctions and sudden depths. At no point can you see the bottom of it.

It is a fault-line in society, of course, a crude symbol, but its scale and daring raises it above that banal concept.

London's streets are at present being ripped apart as its cast-iron Victorian water mains are replaced. We are very tired of linear holes in the ground. There is a rich irony in the fact that Salcedo's version has got us all chattering about the cleverness of, um, a cavity.

Links
Salcedo at Tate Modern: http://www.tate.org.uk/modern/exhibitions/dorissalcedo/default.shtm