This is the 10th year that the curators at Tate Modern have invited a world-renowned artist to fill their Turbine Hall. It's not an easy space to fill, what with it being huge and that, but several of them have had a good stab. Light, sound and vision have been all invoked, with some artists concentrating on the floor below and others on the air above. And it's getting tougher and tougher to think of something original, something that'll make the visiting public go "Ooh, that's different, I like that". This year, after a bunkbed duffer last, I think we've anotherwinner.
This year the theme is darkness, with the appearance of an all-enveloping giant steel boxon stilts. It doesn't look terribly exciting from the second level overbridge, more like an oversized freight container, but then it's not the exterior that's important here. The box's creator is Miroslaw Balka, a Polish architectural artist, and he's the first commissionee to create something the Tate audience can actually walk inside. To find the way in, walk up to the far end of the Turbine Hall and stand at the foot of the ramp rising back into the box. Dark, isn't it?
If the intention is to unnerve, then "How It Is" succeeds admirably. You know there can't be anything too terrible ahead, like a pit of snakes or a rotary machete, because nobody inside the box is screaming. But there could be something unpleasant, or fearsome, or potentially painful lurking in the darkness, couldn't there? And that's precisely what the artist is hoping you'll think, evoking echoes of wartime innocents being herded into concentration camps. Don't worry. Take the installation on trust, and step inside. If you don't want to know what's inside the box until you visit for yourself, stop reading now. The rotary machete (or whatever) will then come as a complete surprise.
It is very dark indeed inside the box. Even if you've heard reports of how dark it is, the pitchblackness of the interior will still surprise you. That's so long as all the participants are playing by the rules and keeping their cameras in their pockets, as requested by a polite sign at the entrance to the artwork. While your eyes are attempting to adjust to the darkness, the last thing you want to see is the flash of a camera or the glow of a mobile screen. Needless to say, there are plenty. It's clearly not feasible to tell today's youth to put their mobiles away, because they'd be lost without them. That pinpoint of bright light you can see, that's some selfish twat taking a photo so that they can send dimly-lit facial images to their mates. Please resist the temptation to punch them in the face (even though in the darkness they'd never know who hit them).
As you move forward, you may become convinced that you're about to walk into something. Or more likely someone - walking back the other way and accidentally bumping into you. Relax, it doesn't happen. What's more likely is that you'll reach the far end of this cavernous space without noticing. It's very hard to judge distance in the dark, or at least to match how far you've walked with the length you saw so clearly on the outside. The best indicator that the end is nigh is an increased level of chatter immediately ahead, because everybody stops at the far wall to survey their position. And yes, the far wall is soft and velvety, so don't be too surprised when you end up with a face full of fur.
And then turn round, and see the box for the illusion it truly is. With daylight trickling in from the windows in the Turbine Hall wall, everything between you and the ramp is now visible in eerie silhouette. A sea of cautiously bobbing heads is approaching - slowly, steadily, oblivious to the daylight flooding in behind. And that's why nobody bumped into you on the way in, because you were as wholly visible to them as they were invisible to you. Soak in the view for a while longer, at least until some fresh arrival blunders onto the very spot where you're already standing and displaces you. Then tread carefully back to the world outside, now more certain of precisely where you're heading. Illuminating, that's how it is.
The Turbine Hall 10 (in order of interestingness) 1) Olafur Eliasson - The Weather Project: A giant radiating orange sun which Londoners took to their hearts  2) Carsten Höller - Test Site: Several slippery twisty-turny metal slides (use at own risk, ouch!)  3) Louise Bourgeois - I Do, I Undo, I Redo: Three thin sculpted towers to climb, with big mirrors on the top  4) Miroslaw Balka - How It Is: Walk into a very dark box, and try not to bash your nose into the furry wall  5) Anish Kapoor - Marsyas: Three steel rings sheathed in a one-piece PVC membrane  6) Juan Muñoz - Double Bind: Stand beneath a low floor as lifts rise and fall through the space above  7) Doris Salcedo - Shibboleth: A big crack in the floor (the filled-in trench is still very visible)  8) Rachel Whiteread - Embankment: Thousands and thousands of white boxes to wander between  9) Dominique Gonzalez-Foerster - TH 2058: Blue and yellow bunkbeds form a futuristic refugee camp  10) Bruce Nauman - Raw Materials: 22 audio recordings to stop and listen to (if you can be bothered)