The Milan Expo 2015 features purpose-built pavilions, housing exhibitions from over 140 countries and private organisations and designed around the overarching theme of Feeding the Planet, Energy for Life.
Occupying a site near the city's Rho Fiera exhibition grounds, which hosts the annual Salone del Mobile furniture fair, the Expo is expected to attract more than 20 million visitors over six months. Officials say that they have already sold more than 9 million tickets.
Many of the pavilions and other structures on the site were completed at the last minute, or still have work remaining, so official photographs are not widely available yet.
But visitors have been using Dezeen's #milanogram2015 tag among others on Instagram to share their first impressions – and almost everything is complete.
Among the most popular structures are the beehive-inspired British Pavilion, designed by artist Woflgang Buttress with engineer Tristan Simmonds; the Japanese pavilion by architect Atsushi Kitagawara with a multi-dimensional facade made from a lattice of wooden blocks; and the Bahrain pavilion by architect Anne Holtrop and landscape architect Anouk Vogel, which was conceived as a continuous landscape of fruit gardens intersecting with enclosed exhibition spaces.
Other early favourites include the Brazilian pavilion, by Atelier Marko Brajovic and Studio Arthur Casas, which features an interior landscape created from a criss-crossing net of ropes that visitors can walk and climb on; the Italian pavilion designed by Roma studio Nemesi with a white lattice facade made from air-cleaning cement; and the Chinese pavilion with its wavy roof designed by New York firm Studio Link-Arc and a team from Tsinghua University.
Despite today's relatively calm opening, the Expo has had a troubled path to fruition. In March this year, architect Jacques Herzog, co-founder of Swiss firm Herzog & de Meuron and one of the original master planners of the Expo site, launched an attack against the project, branding it a "vanity fair".
Herzog was invited by Italian architect Stefano Boeri in 2009 to help masterplan the Expo site, but had walked off the project by 2011 along with American designer William McDonough and British urban designer Ricky Burdett who had also been part of the design team.
In 2014 the commissioner of the Italian pavilion stepped down after being arrested over corruption and bid-rigging claims, and there have been protests against the project by local residents over the amount of money being spent. "No Expo" graffiti appeared across the city in the run up to the opening.
The Italian government has committed more than €1.3 billion (£958 million) for the project thus far, and is spending an additional €3 million (£2.2 million) on a commission for 11,000 square metres of temporary camouflage screens, officially described as "external exhibition elements", to hide ongoing construction work on the site.
At the beginning of April, reports in the Italian press suggested that only nine per cent of pavilions had been completed.
When Dezeen visited the site during Milan's design week – two weeks before the Expo was due to open – a large number of pavilions were still under construction and concrete was still being poured. A number of designers working on the project expressed doubts that the site would be ready on time.
But according to reports from today's opening, on-site issues have now largely been resolved or hidden from visitors.