There's a huge crack in the floor of the Tate Modern. It was put there by the Colombian artist Doris Salcedo, and it's not clear how. Impressively, it runs the entire length of the building, in a concrete floor that, surely, was there before and will have to be there later, when the next artist comes to fill the Turbine hall with something else momentous.

It seems that Salcedo has been inspired by her contact with people involved in the Colombian Civil War. Another work of hers in the gallery (Untitled, 1998) is made up of a chair cemented into the back of a dresser, the two structures flush. Uncannily, it evokes the feeling that something awful has happened here, and that we're shutting it up and papering over it. The redundant, asymmetrical lines piercing the surface also bring to mind a human being with a mutant skeleton, just visible beneath the skin. Another creepy piece (Unland: Audible in the Mouth, 1998) fuses together two long wooden kitchen tables of slightly different shapes; they are made to depend on one another to stand, and to compromise their shapes to fit together. There are long human hairs stretched into the surface, cutting into it with meticulously parallel lines. It seems like a torture device, or what's left over in the torture room. In both cases, the effect is especially uncanny because it inhabits such common objects—like the dresser, table and chairs in my childhood home.

The crack, called "Shibboleth," is still more approachable, more playful. I liked watching children look deep down into it ("It's deeep!" they would say), sticking their hands in, wondering how it could be and what it was like. Like myself, they seemed to like moving along it, looking into different bits, not believing, or expecting, that it's all the same. At kid scale, it's an imaginary landscape, something the mind, or a mental spaceship, can soar over, perch on, and otherwise play in. At grown-up scale, it's still a pleasing perplex, but the sinister side comes through as well: this is a division, a marring, a breakage, a catastrophe, a borderline, and a shibboleth, too: it separates us. Something awful happened here, though this time it's not papered over.

As always, the best show is in the crowd, in the reactions. I satisfied my curiosity by walking the full length of it, checking it, repeating to myself, "Yep, it's a crack all right, a real crack." But as I walked out on the mezzanine level, which overlooks the piece, a woman with kids was coming in. I overheard: "...but it's not a real crack, it's a fake crack."