Bankside revisited: as the first public event in May 2000, London Taxi drivers were invited to a party in the Turbine Hall as part of a clever marketing plan to raise Tate Modern's profile. Since then, what has been happening in London's newest public square, a destination that through the success of the Unilever Series has become a new gauge for public art?

The construction of Herzog and de Meuron's Tate Modern (AR August 2000) was significant for many reasons. Not only would it provide London with a long awaited gallery of modern art, but it would also act as a significant catalyst for the regeneration of the London borough of Southwark--a previously neglected area of the capital's core. By proposing the reuse of Giles Gilbert Scott's austere Bankside Power Station--which narrowly avoided spot listing by English Heritage--the project is exemplary, not only in terms of promoting the creative reuse of grey-building stock, but also as an inspiration to visionary clients to run well-conceived design competitions, with the process bringing a significant new chaptet to the debate on gallery design. (Competition were Ando, Chipperfield, Moneo, OMA, Piano, Alsop & Stormer, Silvestrin/Judd. Future Systems, Grimshaw, Mather, Hopkins and Isozaki.)

Perhaps what was less expected however, was the building's effect on the public realm, and on the notion of public art. While Herzog & De Meuron's 1995 competition-winning scheme made provision for landscaping, creating a bankside habitat for passers-by and gallery visitors, did they ever foresee that a new (unplanned) London square would emerge at the heart of their scheme? Admittedly it is privatized space, but, reminiscent of the Royal Festival Hall foyer, the turbine hall (the original engine room of Gilbert Scott's building) has become a popular 'worth a detour' destination en-route along the Southbank, or as a short cut from St Paul's to Southwark.

As the centrepiece of H & dM's design, the work-with-the-building's-assets approach seemed simple: strip out the machinery, reluctantly replace the rooflight, and insert a bridge and ramp to resolve level changes--creating an iconic space that, through the stewardship of Tate and Unilever, presents artists with a new challenge, more significant perhaps than the now weary Turner Prize. By bringing 'sensation' back to art, without relying on blatant controversy or celebrity status, two years after Rachel Whiteread's Turner Prizewinning House dramatically took art off the wall and into the public realm, H & dM's response had clearly struck a chord.

As described by senior curator, Frances Morris, the relationship between Tate and Unilever has been very successful. Initially proposed in 1997 as a sculpture walk along the Thames, through discussions with the Tate as the Turbine Hall was taking shape, the Unilever Series evolved into an undertaking to sponsor a series of installations--starting in 2000, when the gallery opened, with Louise Bourgeois.

Over the past four years, the series has led to many exciting modes of collaborations between artists, engineers and curators, as procurement, fabrication, lead-in times, health and safety and even crowd control have become necessary considerations in the artist's otherwise solitary process. Even Bourgeois' relatively modest intervention--a series of architectural objects placed within the space--came with its own curatorial challenges, as seven containers of components were shipped from Connecticut to be assembled by a team of local steel fabricators in the hall's east-end. With welding, grinding and heavy use of the Hall's beam crane, the Turbine Hall was certainly made to work hard. But that was just the beginning. Since then the three subsequent works have become increasingly ambitious in scale and technical complexity, demanding larger teams and the creative input of specialist consultants: primarily structural engineers, with Juan Munoz collaborating with Neil Thomas of Atelier One. Anish Kapoor with Arups' Cecil Balmond, and Olafur Eliasson with Switbert Greiner.

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Juan Munoz--who died in 2001 before witnessing the full effect of his work Double Bind--confounded the expectations of his curator Susan May with his response to the Hall's dominant architectural character. Mindful of the number of visitors (which at weekends can reach 15 000 per day), and using H & dM's bridge as a datum. Munoz created a series of architectural situations for visitors to discover and contemplate, at their own pace. Three distinct spaces that, as described by May, played on perspective, illusion, visibility and invisibility: the patterned floor extending from the bridge; the dramatically lit subterranean world beneath; and the magical inhabited space in between--all dynamically tied together by the hypnotizing effect of two lifts locked in perpetual motion.

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While Munoz's lifts addressed the Turbine Hall's height, Anish Kapoor chose to tackle its height by emphasizing the hall's length, with a dramatic membrane structure. Despite a commonly held assumption that Marsyas was a direct adaptation of Taratantara--a previous installation within the shell of the Gatehead's Baltic Flour Mills (AR September 1999), many options were tested in search of a form that would adequately engage the entire volume of the space. These included, two Baltic forms--one on either side of the bridge--a solid sprayed concrete peanut-like form, cantilevered 30m on each side of the bridge, and a pressurized mirrored form, with a bi-focated ballast sack hanging precariously above the bridge. However, after a period of ambitious invention (a process that Cecil Balmond of Arups saw as essential to gain Kapoor's aesthetic trust), it was decided to simplify the idea with two vertical rings to resolve lateral forces, and a third horizontal ring to provide the tension required to create Kapoor's smooth form, wrinkle free.

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Editing ideas was also part of the process undertaken by Olafur Eliasson, when creating his installation The Weather Project. Having eliminated plans to make it rain and snow, to create an interior rainbow, and an ambitious plan to turn the Turbine Hall into a giant fridge, Eliasson became as interested in engaging with the institutional ideals of the Tate as with the space itself. So, while Munoz's work measured space, and Kapoor's visually measured forces. Eliasson chose to measure people through a survey of their perception of the weather. Through this he then set about creating an environment within the Hall that would play on experience and expectation--rather than the spectacle of the object.

This work, with its mirrored ceiling and burning sun, perhaps more than any other, allowed the hall-as-square to truly come into its own, with over two million people visiting the space, to dwell, look and lie down. Seeking refuge from the reality of winter, as if passing time on a lazy summer's eve, people reclined, some slept, and amorous couples embraced, in the haze of the warm sunset, gazing into the mysterious sky above at faint, distant reflections of themselves. Summer certainly came early last year--so what we may ask will next winter bring? There is much more to this space than meets the eye. What, for example, could it sound like? Bruce Nauman's installation opens in October.