You'll probably know that a crack has appeared in the floor of Tate Modern. What starts as a hairline crack in the concrete at the west entrance stretches the entire length of the Turbine Hall, ziz-zagging down the ramp, widening and deepening until it disappears under the far wall 167 metres away.

But no, the foundations of the former power station are not finally giving way. The fissure is the deliberate work of Colombian artist Doris Salcedo and the latest installation in Tate Modern's gargantuan Turbine Hall in the Unilever series of art commissions. It follows on from 2006's giant slides, and before that sculptured shafts, a red horn, a sunrise, mesmerising sounds, stacked boxes, towers and a giant spider: the inaugural commission by Louise Bourgeois for Tate Modern's opening in 2000. Incidentally, this famous arachnid has now crawled back and stands outside on the banks of the Thames heralding a retrospective of its French-American creator, more of which later.

Memorable as all these commissions are, Salcedo's is quite different. Gone is the funfair element of last year; in its place something stark, something that demands we look down, subverting the normal desire to look up and admire the grandeur. The work has drawn mixed reactions. Underwhelming was one view. Another, better put perhaps, is understated. Nicholas Serota, the director of the Tate points out: "You have to take time, encounter it yourself, walk the length of this great sculpture, to reflect.

"This is the quietest, most unobtrusive of any of the eight works . . . a profound and disturbing work . . . that works on the immediate level of the crack' but also on memories, our histories, our whole way of looking at our world."

To Salcedo it represents "the gap between humanity and inhumanity - a bottomless division".

It clearly has political meanings. It is called Shibboleth, after all; a word with Old Testament origins that commonly means a test of membership to, or exclusion from, a particular group or social class. A word used therefore to separate one from another.

Wire is embedded into the cleft of the subterranean sculpture to emphasise this, because wire, Salcedo says, is "the most common means of control used to define borders and divisions".

She continues: "The negative space' created represents borders, the experience of immigrants, the experience of segregation, the experience of racial hatred."

Shibboleth (until April 6) is Salcedo's first public commission in the UK. Before, she was perhaps better known for lowering 280 chairs down the side of the Palace of Justice in Bogotá in 2002 in homage to those killed in a coup, and the ordered chaos she made of a derelict building plot in Istanbul filling it with tumbled chairs.

Chairs also play a part in the later work of sculptor Louise Bourgeois - as do spiders, and not just the 9m-high Maman outside, one of a series of six giant spiders made in the 1990s.

The Louise Bourgeois retrospective, on until January 27 at Tate Modern, displays over seven decades of highly original work by the now 95-year-old sculptor. Broadly chronological and covering distinct stages in her career, the more than 200 pieces include drawings, prints, paintings and sculpture, as well as several of her cells', unsettling works full of symbolic clutter, such as old chairs, mirrors, bits of tapestry, spindles and blood-red spools.

Hers has been an extraordinary career. Born in 1911 in Paris to parents who ran an antique tapestry restoration business and moving to New York in 1938, Bourgeois has always been at the forefront of new developments in art. She studied under Ferdinand Léger in Paris, was visited in her Manhattan home by Giacometti, Bonnard and Brancusi among others, and tapped into (but never joined, says curator Frances Morris) many of the major art movements.

The Femme Maison (housewife') series in the opening room reflects the clear influence of Surrealism. The half-naked, half-house women in the paintings seem to be trapped, trying to hide, or have outgrown their confines.

The woman as carer, nurturer and supporter theme appears again and again in her art; hence the spider motif. Bourgeois explains: "The Spider is an ode to my mother. She was my best friend. Like a spider, my mother was a weaver . . . spiders are helpful and protective, just like my mother."

Her sources of imagery are deeply autobiographical: her troubled childhood, especially her relationship with her tyrannical father, motherhood, sexuality, architectural spaces, cages. Cell (Choisy) 1990-3, centre stage in the first room (pictured), illustrates this completely: a flesh-pink marble model of her childhood home, which also housed the family business (a sign "Aux Vieilles Tapisseries" confirms this), set within a cage, a prison. A guillotine at the front symbolises how people destroy one another within the family, maybe too the desire to annihilate memory. It's all very literal.

Bourgeois gets her revenge, however, in 1974 with The Destruction of the Father. In a claustrophobic spider's den of an installation, Daddy is the dinner apparently. Bourgeois described this "terrifying family dinner table" headed by the father. What can the mother and children do, she asks, then adds: "So we grabbed the man, threw him on the table, dismembered him and proceeded to devour him."

It requires stamina, this exhibition. Time and again it offers up hidden dramas of private and family life reworked into stacks, spirals, cells and increasingly organic-looking objects - some bulbous, some gloopy, seeming to come straight from special effects, and some emphatically sexual. And then it offers them again as fabric pieces made in recent years. It is stuff worthy of the best psychoanalyst - if definitely not for the arachnophobe.