karlpejera: The Tanks at Tate Modern ? review

It may just look like a lot of old concrete, but Herzog & de Meuron's conversion of the oil tanks beneath Tate Modern is a work of art in its own right

When the dust settles on this year of spectacle and recession-proof Olympic construction, of teetering steel pointing to the sky, of swinging cranes, mayoral openings and breathless press releases, of the mighty Shard and the risible Orbit, one of the most memorable works will turn out to be something invisible above ground, and largely the creation of little-known 1950s engineers. This is the conversion of the oil tanks of the former Bankside power station in London ? a clover of three buried concrete cylinders ? into new spaces for performance and exhibition for Tate Modern, which is what the power station became in 2000.

This project is by the Basel-based firm of Herzog & de Meuron, architects of that icon of icons, the stadium built for the 2008 Beijing Olympics ? or Bird's Nest, as the architects don't like it to be called. The stadium set out to be a modern Eiffel Tower and succeeded, impressing its image on global television audiences of billions: of all the attention-seeking buildings of recent years, it commanded as much attention as any.

The Tanks at Tate Modern are a work of uncovering as much as addition, and finding rather than making, and there is not much chance that they will imprint their image on the world's imagination. You may indeed not be overwhelmingly excited by the photographs on this page, as the Tanks are hard to capture with a lens. They are voids rather than objects, things to be experienced rather than seen.

They are not, however, exactly modest. Each has a height of 7 metres and a diameter of 30 metres. Built to hold the fuel that powered the turbines that generated electricity for a large chunk of the capital, and to withstand the risk of explosion, they are thick-walled and capacious, a trefoil of inside-out towers, or a castle in negative.

They have the confident geometry of things made not for appearance but for a substantial practical use, and their concrete surfaces have a raw force. They are impressive just for being there, and the architects' approach has been largely to let their size and mass speak for themselves.

One cylinder has been made into a performance space, another into a room for temporary installations with a preference for those using moving images. The third tank has been subdivided into the functions, such as dressing rooms, needed to serve the other two. Floors, ceiling and lighting have been added as necessary to fulfil their new tasks. On the approach to the Tanks you see an archaeology of patches and slatherings of concrete, marks of the changes and repairs made to the structure over the years.

Unless you are a connoisseur of concrete techniques during the second Elizabethan era, you don't greatly distinguish between the changes made this year and those of a half-century ago. As in Astley Castle in Warwickshire, you're presented with a composite of past and present, where the joins between the two are not strongly marked. For an architect on such projects, according to Pierre de Meuron, "it is mostly good advice to disappear, as you leave your traces anyway. It looks as if it has been so forever but it was actually different before." The urge to dramatise what is already impressive has been avoided.

Rooflights could have been added, such that daylight could have gushed into the Tanks and made them into romantic Piranesian ruins, but Jacques Herzog says this "would have been a trap ? it becomes kitsch if you make too much of it". The way that he and de Meuron have designed it is to stress that "it is really an underground space". Each tank has "a door and that's it. It's just very straightforward."

The effect of this approach is to flatten the experience of the Tanks, such that their scale grows on you rather than amazes you all at once.

It also allows them to function as background to the performances and installations they will hold, rather than be the star turn in themselves. They will only be completed by the things that will happen in them, which is as it should be. And if you want drama there is no shortage of it in the Turbine Hall next door, or in a multi-storey twisted brick pyramid that is about to rise over the Tanks.

It's tempting to see the fondness of digging and revealing as a sign of our less shiny times, and it can also be seen in the same architects' Serpentine Pavilion, and their plans for renovating the giant Park Avenue Armory in New York, but as the Tanks and the pyramid together will cost �215m, this new modesty would have to be taken at a purely symbolic level. It would be more accurate to see it as a continuation of the original idea of 1994, when it was first decided to make an old power station into the Tate's new gallery of modern art.

The plan was derided at the time, as a backward-looking, heritage-obsessed failure to take the opportunity to commission proud new buildings, but the success of the Turbine Hall showed that it can be as powerful to use and modify found spaces as make new ones. The Tanks, meanwhile, remained a great secret place, buried beneath the grass behind Tate Modern. It was a particular obsession of Tate's director Nick Serota (who liked, evidently, to park his tanks under the lawn) to bring them into use.

"I want those oil tanks," he told his architects, "even if I don't know how to use them," and eventually art based on performance and moving images provided the reason, and Tate's genius for fundraising came up with the cash. And so an intriguing new republic has been added to Tate's imperium, or a new district to its city of art. Visitors will now have a reason to turn right after passing through Tate Modern's main entrance, where at the moment all the interest is on the left.

There is an obvious danger in pursuing space for its own sake, which is that whatever goes in it feels like a pretext, a token gesture embarrassed by its setting. There is also a danger that Tate's relentless expansion will bloat it, and make it in the end an exhausting barrage of over-spectacular presentations. With an exhibition area of 1,800 sq metres, the Tanks are more than half as big as the not-small Turbine Hall, and exceed the display space of entire regional galleries, such as the Turner Contemporary in Margate.

But at this point, not yet tested by use, the Tanks feel credible.

There seems to be a purpose to their intended programming with film and performance, and they are distinctive enough ? exceptionally so ? to be rather more than another dollop of space in the Tate's halls. In the end, these underground chambers are simply extraordinary places, of a kind that would not now be invented from scratch. It would have been a shocking not to make use of them.

The Tanks at Tate Modern, London SE1 open on 18 July with Art in Action, a 15-week festival of performance and installation art

Rowan Moore