"Alles ist Kunst" ("everything is art"), proclaims a painting on the wall of a disused brewery in Dortmund, a city in the industrial Ruhr region, recently transformed into an art gallery. It's not a bad slogan for the Ruhr's year as European Capital of Culture - the most unlikely capital of culture that anyone in Germany could conceive.
The industrial powerhouse of the Second and Third Reichs and of the West German Wirtschaftswunder, the Ruhr is Europe's third-largest conurbation. Only London and Paris are bigger but, unlike those cities, the Ruhr has no downtown. This enormous area consists of 53 separate towns and cities, merged by a century of heavy industry into one vast, unsightly urban sprawl. It was founded on coal and steel, but these traditional industries have virtually disappeared since reunification. Unemployment is high and the population has been shrinking. Surely the last thing the place needed was a year-long artistic jamboree?
That was more or less my point of view when I visited the Ruhr a year ago. Although I found much that impressed me - redundant mills and factories reborn as fashionable museums and galleries - I very much doubted that many people would pay good money to come here.
A year later, happily, it seems I was mistaken. With more than eight million visitors so far and a month left to run, Ruhr 2010 is likely to overtake Liverpool 2008 as Europe's most popular cultural capital season so far. The number of foreign visitors has nearly doubled, with the British and the Dutch leading a crowded international field. So why has the festival succeeded in such an improbable place?
From where I am standing (beneath the huge, revolving neon "U" of the Dortmunder Actien-Brauerei), it seems that Ruhr 2010 has prospered by refining familiar landmarks rather than trying to build new stuff from scratch. The Dortmunder U has always been the unofficial icon of this gritty city. Now, it's a brand new arts centre, rather than an abandoned brewery.
Likewise, the old Nordstern colliery in Gelsenkirchen is now a sleek new studio and office space. The Swiss architects Herzog & de Meuron have designed a swish new extension for the Küppersmühle mill in Duisburg - now a contemporary art museum with work by German modern masters such as Joseph Beuys, Georg Baselitz, Gerhard Richter and Sigmar Polke. Norman Foster has revived Duisburg's old harbour - once the biggest inland port in Europe. Against all odds, a post-industrial eyesore has become a pleasant place to work and play.
The central hub of Ruhr 2010 (in so far as the Ruhr has any sort of centre) is Essen, a workmanlike city with a population of nearly 600,000, which is about the same size as Dortmund. Its cultural focal point is the Folkwang Museum, once described (by a co-founder of New York's Museum of Modern Art) as the most beautiful gallery in the world, but the most arresting building is the monolithic Zeche Zollverein - once Europe's biggest coal mine, now a thriving museum and performance space, also redesigned in part by Foster.
Ultimately, however, the success of Ruhr 2010 is not down to architects or curators - it's a feat of imagination by the people who really matter: those eight million paying punters who travelled here this year, confounding all predictions that Ruhr 2010 would fail. The derelict buildings of Europe's rust belt have become its most vibrant modern arts venues but they would be empty without an audience alive to arts events that reflect the way we live today.
I finished my tour of the Ruhrgebiet at Bochum University, a gigantic concrete campus that towers over the surrounding suburbs like a cross between the Star Wars mothership and a run-down council estate. A few years ago, this brutalist relic would have seemed ugly; today, it feels like a trash-aesthetic period piece. Inside is an exhibition featuring photographic portraits of 100 local people from 100 different countries. This show is called "The New Pott" - a pun on "melting pot" and "Ruhrpott", the Ruhrgebiet's local nickname. Multikulti ("multiculturalism") may be a dirty word in more conservative parts of Germany, but here in the Ruhr these new Germans represent the region's best hope for the future. A century ago, coal and steel brought a flood of immigrants into this gritty region. Now that those industries have gone, the Ruhr must begin again.