Hardly anybody visits football grounds for their architectural charm. Most of these edifices have simple, short-back-and-sides designs. Even well-designed ones are ignored. Munich's Olympic Stadium, for instance, has a half-open roof supported by pylons that looks a bit like a giant tent. The architect, Gunter Behnisch, who had begun his architectural studies as a prisoner of war in Britain, had originally shown Munich's town council a lady's stocking stretched across some sticks. Yet on that June afternoon in 1988, I doubt that a single Dutchman noticed. The architecture has never been the point. Even the celebrated Swiss architect Jacques Herzog, who, with his partner Pierre de Meuron, won the Pritzker Prize, the profession's Nobel, told me: "The stadia I love — Anfield or Old Trafford in England — are ugly on the outside. When the ground is packed, the people become the architecture." That day in Munich, that's what we were.

What you retain from years of visiting stadiums are those moments when the crowd expresses a communal emotion that is either startling in its intensity, or plain surprising. Like the time in Newcastle in 1998, when one of the most partisan crowds in football gave an ovation to the teenage Liverpool striker Michael

The stadia I love are ugly on the outside. When they are packed, the people become the architecture — JACQUES HERZOG, architect

Owen for scoring a hat trick against their team. Or the ferocity of 100,000 Catalans in Nou Camp when Real Madrid visit: waving Catalan flags and raging against Real as if this were still the 1940s, Real were still General Franco's team, and Catalonia were still subjugated. Or, for sheer venom, an "Old Firm" match between Glasgow's crosstown rivals, Rangers and Celtic. A football ground is a good place to witness hard feelings, but nothing beats the Old Firm game, which is best described as "90 Minutes of Hate." Protestants traditionally support Rangers, and Catholics, Celtic. The one time I went (never again) I stood at the Celtic end, listening to the people around me chant, "Oh Ah, Up the Ra!" in praise of the i.r.a., while the Rangers fans facing us sang "Noooo Pope of Rome!"

I used to think football grounds were the place to uncover Western Europe's suppressed ethnic, religious, regional and class tensions. Then one day in Glasgow I met a Celtic fan named Roddy McKay who cheerfully admitted shouting the most outrageous anti-Protestant abuse at Old Firm matches. He had even named his second son after the entire Celtic team of the day. ("The subs wouldn't fit on the birth certificate," he grumbled.) It sounded like the usual story — except that this man was married to a Protestant. Just after his wife gave birth, he had sneaked to the town hall to name his son. The poor woman, who supported Rangers, kicked the sitting-room door off its hinges when she found out. McKay showed me a picture of his son at two days old, dressed in the Celtic home shirt, in the arms of an elder brother who was wearing the Celtic away shirt. "Put it this way," McKay crowed, "he'll never play for Rangers."

To this man, who does not believe in God, the Old Firm game is no longer about religion. Nor is it to many other Old Firm fans: almost half the Scottish Catholics who marry now do so across religious divides. In other words, Celtic and Rangers fans may still shout sectarian slogans at football matches — and some still take their bigotry beyond full time — but most no longer mean it.

And this is true all over Western Europe. In the past, the passions on display in a European football stadium really did reflect religious or class or regional sentiments. Just as Barcelona used to stand for Catalan nationalism, so the Milan-Inter derby set the migrant working classes against the city's own middle classes, while the Dutch in 1988 still carried around a war trauma about Germans. But today, these passions have weakened. Europeans have ceased to believe in God, class divides have narrowed, and while there is now more regionalist chest thumping than ever, it is hard to be quite so fanatical about it now that countries like Spain are decentralized democracies and regions like Catalonia could choose independence if they really wanted.

So when Barcelona fans wave Catalan flags, or Glaswegian fans sing sectarian songs, they are reaching for traditional symbols to express a football rivalry. For that father in Glasgow, his feelings about the city's football clubs were stronger than any sectarian feeling he brought to the game. The chants in a football stadium today are no longer proxies for other passions. Football has become a cause in itself.

And the new stadiums are no longer built as afterthoughts — barns to house the devoted. Rather, they are starting to become as beautiful as cathedrals. Herzog says that his Basel football stadium, which he built at the same time as creating London's Tate Modern art gallery, "was the first of the new football stadia to be built by a famous architect. The other European grounds were strictly commercial projects." His firm, Herzog & de Meuron, is now building a new stadium in Munich for the 2006 World Cup. The stadium will glow on the outside when there is a match on inside. "The idea is that the energy of the people, of the sportsmen, surges outside," explains Herzog. You can imagine, centuries from now, tourists with guidebooks coming to study the ruins, trying to understand what it was that got 21st century Europeans so worked up.