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It doesn't seem so long ago that contemporary visual art was a rather retiring field of activity.
Modern art galleries were places of quiet contemplation and reflection for serious-minded souls. You probably didn't take the kids and there was always a risk of being slightly bored.
Art as a rule didn't try and mix it with the wider world, or become a focal point for big gatherings, or - God forbid - a visitor attraction.
How times have changed. In the past 15 years, contemporary art has become famous: not famous for being either bizarre or ridiculous, either, but (like properly famous things) famous for being famous.
And one reason for this, among many, is that it has lost its inhibitions about size and the outside world.
It got big in every sense and sought public attention. Many of the most eye-catching artworks of recent years have been outdoors: Antony Gormley's Angel of the North topping the bill, Rachel Whiteread's House in east London and the occupants of the fourth plinth in Trafalgar Square - works that have attracted far more notice than public sculptures ever used to and have spawned a new generation of clones.
But, at the same time, another genre has been born: the big indoor art spectacular. Again, Gormley is a leader, with his massed body-sculptures in the Baltic Arts Centre on Tyneside and, lately, with his great chamber of fog in the Hayward Gallery.
But the real home of these shows is Tate Modern. Ever since its inauguration in 2000, it has hosted a succession of colossal installations. Sponsored by Unilever, they appear, each autumn, in its vast and cavernous Turbine Hall.
The gallery opened with Louise Bourgeois' gigantic spider, Maman, looming over the central mezzanine. It was followed by some even more sublime and stupendous exhibits. There was Anish Kapoor's Marsyas, a cross between a trumpet and an internal organ, its bright red skin of tightly stretched PVC membrane opening into deep orifices, filling half the length of the Hall. Olafur Elliason's The Weather Project took things a step further. Here, a great baleful solar disc of sodium light hovered high at the end of the Turbine Hall, bathed in mists, with a huge ceiling mirror doubling the visual volume.
It set a still-unsurpassed standard of theatricality. People gazed into the light for hours, basked and took picnics on the gallery floor beneath it as if it were a real sun.
Last year, there was Carsten Holler's Test Site, five great glass-and-metal "helter-skelter" tubes, descending, looping and twisting through the hall. It was probably the most popular of the shows, with queues of people waiting to take the plunge and be delivered, seconds later, rattled but smiling, on to the ground floor.
When Tate Modern opened it was compared to a cathedral. Now it seems like a fun fair. And though it always keeps a straight face, a fun-fair reputation could do no harm to its spectacular attendance figures.
This month Shibboleth, by the Colombian artist Doris Salcedo, has opened to the public. It is a more sober work, but absolutely up to standard.
The artist has emptied the Turbine Hall entirely, exhibiting nothing but a deep earthquake crack, literally cut into the concrete floor underfoot. It begins just inside the entrance, as a barely visible meandering rivulet of fissure. Gradually, as it proceeds, it opens and deepens and begins to zig-zag wildly across the floor.
It's an extremely well-achieved effect and to imagine how it was made is another source of wonder. There is no sign at all of heavy work - it's as if the crack really had just appeared, overnight. And, when you peer into the crack at your feet (it's never more that a foot wide), it reveals realistically moulded rocky walls on either side, going down to some depth, before they close together with still a hint of darker depths beneath.
It would be an exaggeration to say that the ground under Tate Modern seems to be literally breaking apart. It is not a frightening or disorientating illusion.
Those rocky innards are obviously a crafted formation - not what would really appear beneath the museum. What's more, the crack's overall path and shaping are rather too neat to be natural. There is, in fact, a slightly dead-pan comic aspect to its sharp and zippy zig-zags.
But the thing about these amazing art spectacles, of course, is that neither the artist nor the art establishment will ever be content to let them be simply amazing. That would be too much like fun and not enough like art.
Even last year's helter-skelters, quite obviously a form of fun, were deemed also to engender a state of hysteria, loss of self, almost madness in the user - which, if true, is also true of any white-knuckle theme park ride.
The rule seems to be: the bigger the spectacle, the more it's in danger of seeming simply a spectacle and the more inexpressibly significant it must be in compensation, so to speak.
I can hardly tell you all the things the crack in the Tate Modern turns out to mean. But if you consult the accompanying Tate literature, you'll find it holds a real festival of the interpreter's art.
Take any thought that derives from free associating around the words: crack, fissure, cut, gash, abyss or seismic shift.
You'll find someone saying that this is what the work's about, with special reference to the "War on Terror", or neo-colonialism, or the social fabric of London, or museums. For example: "The scar that this Gargantuan work constitutes is, in the first place, a literal one: the trace of deep pain ... a tiny portion of a cut that really runs through the entire world."
Nor is the artist standing back. Her title, Shibboleth, refers to an Old Testament story about a password, a word one tribe can pronounce and another can't. In other words, she emphasises the "dividing-line" aspect of the crack. But don't let that cramp your style. What does she know?
But in a sense, a work like this really is a dividing line in that it divides up its audience.
On the one side, traditional arty types believe that things in art galleries should, at all events, mean something very serious. On the other, the new audiences are attracted first and foremost by an eye-boggling spectacle.
And then there is the Tate itself, doing its best to stand astride the gap that it has opened up, keeping one foot firmly on either bank, trying hard not to do the splits or fall down the middle.
