Expo 2000, which opens today in the northern German city of Hanover, promises visitors an experience of "the whole world symbolically assembled in one place". And with 174 countries and international organisations participating, that piece of hype is only a slight exaggeration.

Inevitably, there were drop-outs, such as Algeria, Congo and Yugoslavia, but most regrettably the US. Despite being the richest nation on the planet, it expected private-sector interests to finance a US pavilion.

Nearly 150 years after the Great Exhibition at London's Crystal Palace in 1851, and a decade after the reunification of Germany, Expo 2000 is the first such jamboree to be held on German soil. It also aims to pave the way for a new relationship between people, nature and technology.

An international team of "scenographers", including stellar French architect Jean Nouvel, designed the Expo's main theme pavilions exploring ideas for survival in the mega-cities of tomorrow, as well as the future of work in a globalised economy dominated by digital communications and hyper-mobility. The site is vast. At 160 hectares (384 acres), it is about the same size as Monaco; a disposable cardboard chair, specially made for the Expo, is likely to be a big seller.

With so many pavilions competing for attention, the site is a real architectural zoo. Pride of place, predictably, goes to the German pavilion on the main plaza - 12,000 sq m of floor space on four levels fronted by a concave glass facade, 130 m long and 90 m wide, under a floating concrete canopy.

Some of the national pavilions are quite predictable, others weird and wonderful. The three Baltic states represent a microcosm of what's on offer: Lithuania's pavilion resembles a big yellow Dustbuster, Latvia went for an inverted thatch roof while the open-topped Estonian entry features what looks like a forest of outsize carrots.

Japan's pavilion anchors the western end of the site. Designed by Shigeru Ban, famous for a quite literal curtain-walled house, its structure is made from bamboo-style cardboard tubes, covered by an undulating white roof of translucent acrylic-coated translucent paper and crammed with the latest Japanese technology.

Given Expo 2000's sustainability theme, other countries have also made good use of renewable materials. Spain, for example, is represented by a huge block of cork, which turns out to be wafer-thin, while cork features more substantially in the eclectic Portuguese pavilion along with smooth pink marble and colourful azuleja tiles.

The Hungarians have a timber construction standing four storeys high. It really cuts a dash, resembling two upturned hands with a sail canopy suspended between them.

Switzerland's pavilion, designed by Peter Zumthor, is even more startling because it is not a "building" at all. Instead, it consists of timber beams, stacked nine metres high without a single nail or screw to create a maze which resonates with the sounds made by accordion, dulcimer, saxophone, Alpine horn and the human voice itself.

With no "exhibits" in the accepted sense, other than a pair of three-storey oval-shaped pods for visitors to relax and enjoy some Swiss hospitality, the pavilion itself is "the event". And though it seems quite unstable at first sight, the whole thing is held together by steel rods and springs to ensure solid structural stability. This strangely evocative structure-of-sorts, reminiscent of a stroll through Brooks Thomas, will be dismantled and re-erected at another site as an architectural masterpiece, according to the Swiss.

By contrast, the much talked about Dutch pavilion by Rotterdam-based MVRDV Associates is orgiastically over-the-top. Layered like a badly made club sandwich, it manages to be both organic and deconstructed at the same time. It is also larger than any of the other free-standing national pavilions, apart from Germany's. This very bizarre structure rises to a height of five storeys from a crude concrete base and the top two floors are supported by tree trunks. Live trees are trapped like densely-packed prisoners under fluorescent lighting on one of the intermediate floors, and the whole thing is festooned with decks of flowers and flying staircases.

Apparently, it is meant to represent the Dutch facility for managing the landscape. It is surrounded by a large garden full of what seem to be weeds planted in black gravel, like a volcanic beach. Wind turbines on the roof may symbolise alternative energy, but what about all the energy wasted in its creation?

The Finnish pavilion next door could not be more different. It is just so cool. The front facade is done in silk-screen printed glass, which looks like ice, and the side walls are in dark, heat-treated timber. Inside, there is a copse of 90 silver birch trees, all brought from Finland, complete with the mossy undergrowth of a forest floor.

Five timber bridges cross the copse at different levels between a pair of oblong four-storey buildings on either side, one for entertainment and the other for marketing Finnish technology. A radar system shows how many people are in the pavilion at any time and the temperature can be controlled from Helsinki via the Internet.

Designed by an architect couple in their early 30s, Antti-Matti Siikala and Sarlotta Narjus, the pavilion was the winner of a competition that attracted 141 entries. It will be one of the permanent buildings on the Expo site and, in the meantime, it is bound to become one of the most popular attractions in Hanover.

Children will have to be dragged out of the animated exhibition, which is simply wonderful, while their parents will want to linger in front of a 23-metre panorama of a Finnish lake in the morning mist, with all the sounds and smells as well as computer projections of dragon-fly, pike and even swans taking off in the distance.

Finland's pavilion brilliantly outshines all the rest, including Denmark's offering, which consists of several "islands" in water - a cube, a dome and a pyramid, plus incongruous Dali-esque sculptures - and Britain's pathetic effort, which looks like a DIY superstore with red, white and blue dots. The French pavilion is not much better.

Some 35 African countries have been herded into one of the trade fair buildings, as bleak as the Simmonscourt pavilion at the RDS, with outsize surfboard sails painted by their artists to take the bare look off it. The restaurant here will be serving such regional specialities as kudu, ostrich and springbok steaks.

All over the Expo site, there are more restaurants, shops selling souvenirs and the ubiquitous Irish pub, to cater for a projected 250,000 visitors every day until October 31st.