Blue lagoon: Herzog & de Meuron take the plunge into Barcelona's latest urban clean-up.

A big blue washing-up scourer has landed in Barcelona. Triangular, spongy and with a serrated metal soffit, Herzog & de Meuron's Forum is a truly absorbing building. As part of an ambitious clean-up operation in the Besos river area of north-east Barcelona, the building sits at the end of the recently extended Diagonal, and uses its footprint to resolve the oblique trajectory of the new arterial avenue with the geometries of Cerda's grid and the curious collection of Forum 2004 constructions--the latest of the city's ambitious regeneration projects.

Since the 1992 Olympic games, Barcelona has become an exemplary centre of progressive regeneration, winning international recognition and prestigious awards, and becoming the only city to win an RIBA Gold Medal in 1999. Heralded (curiously in its own publicity) as the Manchester of Catalonia--'a city that never stops'--current plans focus on Barcelona as the city between two rivers: the Llobregat to the south, where significant port and airport expansion programmes are proposed, and the Besos to the north. Here the recently extended Diagonal defines and connects four sectors of redevelopment, with the 22@ plan and Sagrera to each side--post-industrial areas being transformed largely for new 'advanced industries'--and the Forum and Glories at either end--new parks anchored in turn by H & dM's and Nouvel's ground-scraping and sky-scraping buildings; two future-scale landmark buildings, one steadily rising to the south (p42), the other (H & dM's 45,000 sq m exhibition and assembly building) nearing the end of its first incarnation as host of the 'Universal Forum of Cultures' events and as a stage for the city's breathtaking 'Barcelona in Progress' exhibition.

At the heart of the Forum site (AR June 2003), Herzog & de Meuron's stratified building organises three layers of public space, all set audaciously above the city's ring road, the Ronda Litoral. Conceived as sponge laden with water (originally envisaged to cascade across the facade from its water-cooled roof), the elevated form finds its level above a compressed and gently sloping terrain--part of an artificial platform by Martinez Lapena y Torres that extends over and around the existing water-treatment plant towards new coastal parks by Foreign Office Architects (with Teresa Gali) and Beth Gali. In its present setting, amid the carnival clutter that often accompanies large public festivals (temporary lavatories, kiosks, signage), the Forum as place fails to convince. However, if the site is truly given over to free public access later this month, the permeability of H & dM's new plaza should be fully realised; a permeability currently frozen by the unfortunate location of the Forum pay-barrier that runs across and divides the new plaza.

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While simple in form, and at first sight alien to its context, the building is as laden with contextual complexities as it is with hydrometaphor. Subtleties that not only maintain H & dM's innovative attitude towards making, but which also demonstrate its desire to create buildings that structure public space through resonances with the existing city fabric. By overlaying conflicting geometries, the building is locked into the city's matrix, as more than 30 contorted patios twist between the two geometric fixes, cutting their way through the otherwise impenetrable form. The building also shifts a degree or two off axis to inflect quietly on the axial force of Diagonal.

Arriving at the building, either from the adjacent tram stop or from the Metro (avoiding the inclination to be diverted through the vile Diagonal Mar mall), the Forum's prominent apex, dramatically heightened by the falling terrain, is a natural attractor. Like a pier or promontory, crowds gather here before being drawn in and submersed beneath the gently undulating and shimmering water soffit. A place where reflections, the play of light and secret views captivate; a place of shelter from the strength of the sun; and a place through which to walk en route to the esplanades, marina and beaches beyond; and of course, somewhat incidentally, a place from which to gain access to the building's two principal public spaces.

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Internally, the Forum has hidden capacity. Concealed physically by the rising terrain, and proportionally by its scaleless articulation of form, it effortlessly absorbs 3200 people in its vast submerged auditorium, and no doubt similar numbers in its raised exhibition space; two distinctly different inner worlds with sci-fi white foyers below and flexible black exhibition space above.

The superstructure of the Forum has two systems, with the exhibition space isolated from the distorted cores that contain entrances, foyers, services and escapes. Supported instead on concrete columns, the 180m triangular grid acts like a bridge comprising two decks; a macro-structure at roof level (4m deep and stiffened by two concrete decks) and the lower micro-structure suspended 6m below.

Throughout the building, conceptual ideas translate to detail with characteristic clarity; internal and external floor surfaces are the same throughout; robust and durable, the distinctive spray-painted glass composite render is applied seamlessly, reminiscent of Kapoor's blue rocks, with expansion joints concealed behind glazing panels or along folds; and motifs are reduced to a minimum, as the triangle nimbly traverses the distorted soffit, and fractured, depressed hexagonal tiles create exquisite undulating ceramic 'carpets' in the lower foyer. Technical ingenuity is also evident, as metallic acoustic panels maintain the soffit's continuity through the auditorium, skilfully fabricated with 0.04mm gauge sheets on composite timber backboards. Bespoke furniture includes generous auditorium seats and sanitaryware.

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Considering the wider Forum site, time will inevitably tell of its success. While Barcelona continues to make the most of its medieval and nineteenth-century streets, squares and parks--sustaining local and foreign transactions--this new area will have to work hard to establish itself. With sites like this (terrain vague, urban no-man's-land) a new kind of urban morphology emerges. Forced to contend with the peripheral residue of Cerda's grid, where vacant and half-filled plots collide with large-scale free-form infrastructures, a more fragmented plan is defined by isolated objects. With the city's command of detail, materials, furniture and follies, much will be learned by seeing how it adopts this new city grain; a piece of future city as radical in form as Cerda's 1870 plan must have seemed at the time.

As for Herzog and de Meuron, their building is fresh, functional and fun; somehow reinventing Post-Modern simile with no loss of Modernist authenticity, and deploying materials practically and expertly with no loss of scenographic flair. A building that is part stage set and part place-maker, a curious mix of permanent and temporal. From their measured, earnest and rigorous demeanour, this building reveals their seriously funny side. Herzog and de Meuron are not slapstick, but perhaps with their quiet wit they have a role as the straight men of architectural humour.

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