San Francisco - Science museums are often a collection of stodgy exhibits and isolated experiments stuffed into grand, overbearing buildings.

But a new one opening Saturday in San Francisco's fabled Golden Gate Park aims to change that, stressing the all-encompassing role of science in the life of the planet and its inhabitants.

As might be expected from a 480-million-dollar structure in the country's eco-capital, the California Academy of Sciences hopes to be known as one of the greenest buildings on earth - a title that is literal as well as metaphoric.

The building's signature statement is a living roof topped with a rolling meadow filled with wildflowers. The entire structure is encased in glass and supported by slender columns.

It has 60,000 photovoltaic cells to produce electricity, walls packed with shredded denim jeans for insulation, passive cooling and heating and special sensors to dim or brighten lights based on the time of day and weather.

It's a stark contrast to the cluster of seven dated buildings that used to house the "Cal Academy," as it was known to generations of Californians.

"Museums are not usually transparent," said architect Renzo Piano, a winner of the prestigious Pritzker Architecture Prize. "They are opaque, they are closed. They are like a kingdom of darkness, and you are trapped inside.

"With the new academy, we are creating a museum that is visually and functionally linked to its natural surroundings, metaphorically lifting up a piece of the park and putting a building underneath."

Under the roof are two massive domes - one housing a state-of-the- art digital planetarium, which includes a virtual reality show that gives visitors the sensation of flying through space.

The other 30-metre-high dome encases a living rain forest filled with more than 40 species of birds and butterflies. A section modelled on the forests of Borneo includes a large cave complete with bats, snakes and scorpions.

A spiraling ramp takes visitors into treetops for a unique view of the kind of ecosystems found in Borneo, Costa Rica and Madagascar.

"Rain forests are just magical, magical places," said Chris Andrews, the academy's chief of public programmes, during a recent tour for journalists, as the sounds of croaking frogs and squawking macaws filled the humid air.

"It's almost as if nature has thrown a party and they've invited everybody, and everybody has come."

In the basement, a massive aquarium features a coral reef, thousands of tropical fish and a huge tunnel tank that allows visitors to peer through the clear glass up to the towering roof.

Elsewhere, visitors can interact with scientists in a working lab. They can even bring in feathers, rocks, leaves and other specimens from their yards and have naturalists identify them.

An impressive African Hall includes 20 live South African penguins, along with spectacular dioramas featuring stuffed lions, gorillas and other animals. There's an indoor swamp with snapping turtles and a rare albino alligator, and a Naturalist Centre with 210,000 books, 25,000 maps and 300,000 photographs.

"Our mission is to explore, explain and protect the natural world. We've placed a lot more emphasis in this facility on the 'protect' part," said spokeswoman Stephanie Stone.

The new approach, she said, reflects that a 21st-century museum should teach about conserving the natural world, not just display it. "We want people to have fun, but also to have a personal connection with nature," Stone said. "We're trying to provide an emotional connection so they have a reason to care."

Some 2 million visitors are expected each year, though the entrance fee of 25 dollars means they will have to spend quite a few hours to get their money's worth. Judging by the reactions of visitors at a sneak preview that shouldn't be too difficult.

"This is going to end up as one of the wonders of the world among museums. I'm really impressed. It's beautiful," said retired doctor William Freedman, as he looked at exhibits on the Galapagos Islands.

"It's amazing," said 7-year-old Maya Clarkson as she exited the planetarium. "I want to do a sleepover here."