I just returned from Expo 2010 in Shanghai, China, the largest world's fair ever held. I brought back the usual types of souvenirs one gets at these globalized gatherings: a Mongolian fur hat, a bottle of Arrack liquor from Sri Lanka, scarves from Yemen, plus items decorated with the ubiquitous Expo mascot "Haibao," who looks like a cross between a water drop and Gumby.

Oh, and I came back with a case of jetlag compounded by information overload. The scale and richness of the experience is enough to induce the modern Expo version of Stendhal syndrome. I traveled with a friend and we spent eight days, from 8 to 12 hours a day, seeing the fair. We saw about half of it. Marco Polo could not have done better.

The Shanghai fair is enormous. Seventy million visitors are expected during its six-month run; it's more than 2 square miles in size and is situated in the middle of China's largest city, exact population unknown but somewhere around 20 million. The Expo site straddles the Huangpu River and is bigger than some countries. It's three times the size of Monaco with the daily population of a Seattleish city.

We happened to be there on a day in late June when the expo hit a one-day attendance record of over 550,000. That means at peak times, the population density of the fair site is more than three times that of Manhattan, according to one calculation.

What's that like? Jostling crowds, not quite sardine-like, but you have to get comfortable with people bumping, pushing, and sharing space. If you see a bench on which to rest, be assured that four or five other people will simultaneously have the same idea and you'll be playing musical chairs for a chance to get off your tired feet or find a scrap of shade. It's hot and muggy, "plum rain season" they call it.

You'll be alternately drenched in sweat or tropical showers, not unlike a Seattle June if you ramp the temperature into the high '80s and dial in humidity at 98%. Your need to keep hydrated will be extreme.

Fortunately, Coke, water, and beer are widely available and cheap. I'm grateful to the Danish Pavilion for selling cold Carlsburg which I sipped while strolling through their exhibit. The Danes also brought Copenhagen's "Little Mermaid" statue to exhibit in Shanghai, but even she looked wilted in the heat, despite having her own swimming pool.

Shanghai is the first Expo hosted by China and has attracted over 180 countries and exhibitors including scores of cities, provinces, autonomous zones, and non-governmental organizations (NGOs). It has been described as modern China's "coming out party," and no expense was spared. China spent more on the Expo (around $50 billion) than it did on the Beijing Olympics.

The country leveraged the fair to remake an old industrial area in Pudong, and the Expo is full of infrastructure that will live on: a massive government office, a flying saucer-shaped performing arts venue, a new street grid, and subway stops. Post-Expo highrises go without saying. The Expo is a prelude to what has become business as usual in Shanghai: massive new development that makes our South Lake Union look like a pocket park.

The site is dominated by the China Pavilion, a massive inverted pyramidal structure that looks like it's made of red laquered wood built out of Chinese Lincoln logs. It's been called an "oriental crown," and it screams tradition against a city skyline that sports numerous modern, experimental towers and skyscrapers. Walking the Bund district, where Shanghai's surviving old colonial edifices contrast with existing expo-like structures like the Pearl Tower (Shanghai's "Space Needle"), you get the sense that the city itself has already been heavily influenced by futuristic fair architecture. While the city's modern towers loom over the colonial past, China's new "crown" is less derivative. It is China boldly being China.

China Pavilion will undoubtedly take its place in the pantheon of legacy fair structures, which include Paris' Eiffel Tower, Montreal's geodesic dome, Osaka's Sun Tower, and the Space Needle. The best of these exemplify a kind of reach-for-the-sky ambition, but all embody a sense of historical momentum, a blend of tradition and aspiration. That, in fact, is what makes a great fair. Through technology, architecture, and art, Expos and their symbolic structures are expressions of national or global power and hope. They are "motive" rather than votive objects.

The 19th-century American historian and writer Henry Adams found himself bewitched by turn-of-the-century world's fairs and in his Education of Henry Adams talks about his impressions of at least three, ranging from Chicago's Columbia Exposition of 1893 to the Paris Expo of 1900 to the "Meet me in St. Louis" fair of 1904. In Paris, Adams became fascinated by the power of dynamos, and saw in them a limitless force that would propel us in the future. He speculated that this was a new force replacing other cultural forces, such as religious ones, that had fueled Christian civilization for centuries. He intuited that the currents that built Notre Dame cathedral were different from those brightening the world of Thomas Alva Edison.

You can feel such a force at the Shanghai Expo. It is the power of China, but expressed in a particular way that has global resonance: urbanization. The theme of the Expo is "Better City, Better Life," which is to say modern improvement as seen through the lens of urban development. This is the emerging China, in a world that must accommodate billions of new people and, more importantly, consumers. Yet it must try to do so in a sustainable way.

Many of the pavilions address sustainability and urbanization head on, but the fair itself is an example of both the opportunities and the dilemmas. The fairgoers are 95 percent Chinese, and most arrive via bus or train, good for the carbon footprint. Yet the fair itself is the product of a massive building effort involving who-knows-how-much concrete, all for a rather ephemeral purpose. There is an obvious tension between the world expo idiom and green messaging, and that's been true since the first eco-themed world's fair was held in Spokane in 1974.

One of the most popular pavilions, and the most expensive national pavilion at the fair, is the Saudi Arabian showcase. The Chinese visitors stand in line for up to eight hours for a chance to see inside the Saudi "moon boat." The line snakes along in the Shanghai humidity and heat for up to an estimated 4,000 meters — that's a 2-and-a-half-mile-long line! It curves back and forth like a giant colon. Crowds are kept under control by squads of goose-stepping police officers.

The pavilion is a kind of paean to oil, from below resembling a looming oil tanker with an oasis of date palms on top. The structure reflects the importance of the Saudi-China relationship: It's a "gift" to the Chinese at a cost of some $160 million. The relationship between big oil and fairs is likely to continue. In addition to major corporate sponsors like GM and Chevron, big bidders for future expos include oil-rich Houston, Texas, and Edmonton, Alberta, the Texas of the Great White North.