They like to think big in the Windy City. And here's the art to prove it

The centrepiece for Chicago's ambitious Millennium Park was unveiled yesterday by its creator, sculptor Anish Kapoor. David Usborne examines a city mayor's $400m vision

The vision of Anish Kapoor, the Indian-born sculptor, has always been big. Take, for example,Marysas, his giant piece comprising three steel rings connected by a fleshy membrane of blood-red PVC that filled the Turbine Hall of the Tate Modern in 2002. But now his ambitions are global.

The first details of his latest undertaking - the colossal, polished steel centre-piece to the prestigious new art park on the shores of Lake Michigan in Chicago - were released yesterday. The only remaining challenge is transporting the 125-ton sculpture from the San Francisco studio,where it has been assembled to the Windy City.

Millennium Park is a project of daunting scale. First conceived in 1997 and championed by Richard Daley, Chicago's Mayor, it has gradually taken shape just outside the city centre, alongside Michigan Avenue and adjacent to the Art Institute of Chicago. Partly superimposed over what used to be Grant Park but expanded to cover 24 acres of land, some of it sits astride sunken train tracks. By the time of its opening in the summer, the park will have cost in excess of $400m (£222m);more than twice the figure originally envisaged. At its core will be an outdoor music pavilion, designed by Frank Gehry, whose other buildings around the world include the Guggenheim Museum in Bilbao. There will be a modernistic fountain, complete with video screens, by Jaume Plensa, the Catalan designer.

The funding of the park has been an unusual collaboration between the city and private funding. A coalition of donors, spurred by civic pride, has contributed $120m, mostly to pay for the individual artworks.

No fewer than 48 individuals, 20 corporations and 12 foundations have each granted $1m or more to the park's birth. Among the private benefactors are Oprah Winfrey, the television star, and John Bryan, the retired head of the Sara Lee baked foods corporation.

But ballooning costs and repeated delays associated with the park have made it the butt of repeated criticism in the media. As its name suggests, it was meant to open at the start of 2000.

It should be completed in late July, by which time the Kapoor work should be in place. None of the brickbats against the park have ruffled Mr Daley, whose political grip on the city is unassailable. He has vowed that it "will become one of the finest recreational and cultural spaces of any city in the world" and will reinforce Chicago's reputation as a centre of contemporary art, as well as the cradle of modern office architecture in America.

Kapoor's sculpture, once it is installed in one of the park's gardens of lawns and trees, is likely to become a prized cultural icon of the city.

It is Kapoor's first public piece in the United States but it is unlikely to be his last, with discussions under way for more public commissions on this side of the Atlantic.

The task of building the Kapoor sculpture, which has been partly sponsored by the SBC phone company, was given to Performance Structures in Oakland, on San Francisco Bay. Its elliptical form is created from welding together giant steel plates, each 120 inches wide, bending them to fit the curves demanded by the artist and then polishing them to create the surface of a mirror.

The sculpture, as yet unnamed, will be 66 feet long, 32 feet tall and 47 feet wide. The bean - Mr Kapoor has also likened it to a giant drop of mercury - has an instep that creates a space for the public to stand beneath, as if they were in a reflecting cave. They will be invited to look up to see their distorted reflections in its surface.

This week, Kapoor, who is based in London but was born in Bombay in 1954, visited Oakland to inspect the sculpture's progress.

Ethan Silva of Performance Structures told TheArtNewspaper. com: "What we're trying to do is not natural to the materials. To my knowledge, no one has ever made an optic this big."

He added that Kapoor "is very particular, but we are too". The final polishing of the work, to create its reflective sheen, will be taken care of in Chicago.