How invisible should a major public building be How invisible can a major public building be These thoughts must have crossed Renzo Piano's mind when he made his first sketch for the newly opened California Academy of Sciences in San Francisco. That drawing revealed nothing more than an energetic and undulating, pencil-thin roof, yet this was to be the key to this special design.Beneath this hypothetical roofline, the building that now houses a planetarium, several slices of rainforest, a colony of seabirds, giant reptiles, classrooms, bats, 18 million pickled animal specimens, chameleons, temporary exhibition spaces, geckos, auditorium, frogs, cafeteria, lungfish, and millions of visitors each year was initially left undrawn. The roof would determine the whole. The main body of the academy was to be as immaterial as technically possible. The details would be filled in later.Why was this Because the whole point of the California Academy of Sciences, founded in 1853, is its research into and celebration of the natural world, and especially of biodiversity. What better way to highlight these endeavours than to build a new home that does as little as possible to obstruct the trees and lawns surrounding it in the Golden Gate Park, a home that touches the ground with the delicacy of a ballerinaThe steel columns that support the enchanting green roof of this parkland pavilion are so thin, they must be held in tension by long wire cables. These not only permit the interior of the museum to be as transparent and as free from structural intrusion as possible, but, in the event of an earthquake, should allow the building to sway safely like a ship weathering a storm at sea.It was, in fact, the Loma Prieta earthquake of 1989 that determined the academy's need for a new home in the first place. Until then it was housed in 11 buildings in Golden Gate Park, dating from between 1912 and 1976. Some were badly damaged by Loma Prieta. Since then, several much-loved old buildings in the park have been restored, including the late 19th-century Conservatory for Flowers - the biggest building of its type in North America - while such dazzling new designs as Herzog and De Meuron's copper-clad De Young Museum have added fresh charms to this vast outdoors space, claimed from Pacific dunes in the 1870s.Facing the De Young Museum across the park's open-air music concourse, the Academy of Sciences has been an instant crowd-pleaser. Its clear glazed walls reveal some of the attractions inside, notably the two great spheres of its planetarium and artificial rainforest. From the outside, you can see straight through the building into the park from almost any angle.Inside, the academy is as bright and airy as it is generously proportioned and clearly planned. The central lobby is lit by a over-storey whose windows actually open far rarer than it should be, and protected from the sun by automatic blinds. Throughout most of the year, the building fills with ocean breezes sped up and slowed down, deliberately and to subtle effect, by the roof's artificial hillocks. Sunlight casts shafts of light and shadows across walls and floors.The roof really is an extraordinary thing. Fortunately, there is a viewing platform from which visitors can watch it grow. Here Piano and Frank Almeda, the academy's botanist, have planted 1.7m native California plants. Beach strawberries, self heal, sea pink and California poppies are already attracting hummingbirds, bees and endangered species such as Bay Checkerspot and San Bruno Elfin butterflies.The roof does more than attract wildlife; it also helps to keep the building's interior 6C 10F cooler than a conventional covering would, while dampening noise in the galleries. And because it is surrounded by a band of 60,000 photo-voltaic cells, the academy will use around a third less energy than the maximum allowed by San Francisco's strict laws.Seventy-one-year-old Piano, who made his name internationally with the design of Paris's Pompidou Centre with Richard Rogers in the 1970s, has made great strides in the US in recent years. As well as the California Academy of Sciences, he is currently working on, or has recently completed, buildings for the Art Institute of Chicago, the Los Angeles County Museum of Art, the Whitney Museum of American Art in New York, the Isabella Stewart Gardner Museum in Boston and the Morgan Library and Museum, New York. He has also designed a new headquarters for the New York Times in Manhattan. A man who, in his own words, was once one of the "bad boys" of architecture has become a kind of patrician figure among high-minded American cultural institutions.This latest building is an adventure on any level. The planetarium offers the thrill of journeys to the stars, while the rainforest sphere delights with four different habitats on four different levels. Elsewhere in the building, there is a swamp of alligators and a deep tank nurturing a living coral reef. A colony of Cape penguins can be found in a reconstruction of the 1934 Africa House. The scientists themselves can be seen at work through glass partitions.This is, I think, an important building. Built of recycled steel, 90% of it recovered from the old academy pavilions damaged in the 1989 earthquake, and locally sourced concrete, it is exceptionally "green" even before one takes account of its special roof. It is somehow classical, modern and organic in one and the same green breath. Schoolchildren and scientists are at home here, as are the thousands of living exhibits, those millions of rooftop plants and the winged visitors they attract.When Piano was designing his first major building in the US - the Menil Collection in Houston, completed in 1987 - I remember discussing the notion of "soft machine" buildings with him. How could truly modern buildings using the latest technology and materials be more gently related to our senses and with nature There is a long way to go before architects, and their clients, strike the right balance, yet here in Golden Gate Park is an enjoyable, elegant and environmentally friendly building that should be recognised as a key staging post.ArchitectureMuseumsGreen buildingConservationUnited StatesBiodiversityZoologyPlantsSan Franciscoguardian.co.uk &ampcopy; Guardian News & Media Limited 2008 | Use of this content is subject to our Terms & Conditions | More Feeds