Snow bound: in the high backbone of Japan, rusted steel super-strong skin resists winter loads and thermal stresses.

The Niigata Prefecture is to the east of Japan's big island Honshu, and runs from the sea to the high central backbone of the country. In the mountains, up to five and a half metres of winter snow can settle, literally submerging buildings and the even young trees of the magnificent, scented evergreen forests. To allow the public to interpret and investigate the natural world, the Matsunoyama Natural History Museum has been set up on the edge of the forest overlooking mountains and meadow.

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Takaharu & Yui Tezuka have made a building that wriggles, snake-like east-west through the landscape in a brown, almost smooth rusted steel skin. Entered from the south, the snake encloses an exhibition gallery showing natural and artificial worlds, a reception hall, administration, a lecture theatre and, as the snake's head twists round from east to west, a posh cafeteria called 'the culinary arts experience'. A rusted steel observation tower terminates the tail to the east, and is climbed by energetic visitors to obtain magnificent views over forests to the mountains. At key moments in the plan, notably where the snake changes direction, great transparent panels are inserted in the skin, offering marvellous views into the forests surrounding the site. The mullionless transparent expanses are so big that they cannot possibly be called windows; they are almost invisible thresholds between interior and the outside. They reinforce a feeling of heightened reality, enhanced by the strange perspective tricks of the route.

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In winter, the temperature difference between inside and exterior is often very great. And pressure from deep snow can be extraordinary (depending on the nature of the snow, how it fell, and how long it has settled and so on). So the 'thermally stable' plates of rusted steel that form the outer skin are 6mm thick, and are supported on a skeleton of steel I beams. Skin and skeleton are designed to withstand pressures of 1500kg/[m.sup.2]; the equally pressure resistant acrylic panels are 75mm thick. All steel elements are thoroughly insulated. Inside, there is a skin of plasterboard supported by a lightweight inner steel skeleton. This white skin is separated from the main structure by a generous cavity that acts as part of the ventilation and heating system. Warm air is injected along grilles in the polished concrete floors and stale air is extracted through slots in the plasterboard at eaves level. Heat is radiated to the interior through floor, walls and ceiling. In summer, the system can be used to circulate cooling fresh air.

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In winter, the museum projects through the snow with its tapering tower acting as a landmark and sign of civilization; it groans with snow stresses. People look out into the surrounding banks of snow in which a surprising amount of life flourishes below the surface. In summer, the long brown snake slips along the contours of its semi-wild habitat, which is enhanced and intensified by timber paths and a deck by Tadashi Kawamata. From some points of view, the museum seems like a picturesque long-abandoned industrial building, a mine perhaps, in the middle of the countryside. Other aspects in different seasons reveal a cave, a shelter amid the snow, a lighthouse, a welcoming hut in the forest. And of course always an animal: snake or even fox. The museum's complexity of possible readings and spatial events enhance those of the natural world it sets out to interpret.

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