San Francisco's new California Academy of Sciences puts forest, sea and space under one (green) roof

This weekend marks the long-awaited grand reopening of the academy, which is unusual in that it houses an aquarium, planetarium, natural history museum and educational programs under one roof. In commemoration of the very big deal that all of this is, several hundred butterflies were to be released at its Saturday debut in San Francisco's Golden Gate Park, starting two days of hoopla that's set to include music, Chinese acrobats and a Native American blessing.

But the star attraction is the building itself, designed by Pritzker Prize winner Renzo Piano (Los Angeles County Museum of Art, Pompidou Centre in Paris) and poised to be one of the world's greenest buildings.

Call it the "unmuseum," said Gregory C. Farrington, executive director. "Museums are supposed to have thick walls and dark corridors. You're supposed to get lost.

"This," he said "is entirely different. It's welcoming and full of life and light."

The place is spacious, sunlight-soaked, with glass walls surrounding a central atrium, but its crowning achievement is the living roof: a 2.5-acre biotic expanse with seven grassy domes creating a roof-scape of rolling hills and valleys that echoes San Francisco's topography and its seven predominant hills. The steepest peaks carpet the academy's domed planetarium, rain forest and aquarium exhibits.

The museum "is visually and functionally linked to its natural surroundings, metaphorically lifting up a piece of the park and putting a building underneath," Piano said. If you were a bird, you wouldn't even see the museum from above. You'd be mostly concerned with the 1.7 million native plants growing on top (including strawberries) and the various bugs snuggling in.

From an environmentalist's-eye view, the green roof embodies natural cooling and heating systems, water conservation, solar panels and a new habitat for birds and butterflies.

From a kid's-eye view, the rooftop may strongly resemble Teletubby-land. But fans of Teletubbies may be even more interested in the museum below, where the giant Pacific octopus in the Steinhart Aquarium can stretch 7 feet or shrink to tennis-ball-size.