Buried treasures.

Poetically integrated within a mountainous landscape, I.M. Pei's new Miho Museum reinterprets traditional Japanese forms to create a modern armature for the display of precious artefacts.

Set in a sublime landscape of densely wooded mountains outside Kyoto, the Miho Museum was designed to house the Shumei Family Collection of precious artefacts from ancient civilizations in the Middle and Far East. Established in 1948 by Mihoko Koyama, the diverse collection includes vessels, statuettes, bronzes, jewellery, textiles and an exceptional array of Japanese tea ceremony objects. Designed by I. M. Pei, the new museum is on a steep hillside in a nature reserve. Prefectural and national conservation requirements sought to limit disruption to the landscape, so over three- quarters of the 17 400 sq m building is buried underground. The relationship of the museum to its topography is also influenced by early Chinese and Japanese landscape painting, in which a complete narrative or image is never fully revealed, but glimpsed in fragmentary ways. In particular, Pei was inspired by the traditional Chinese tale 'Peach Blossom Spring' by Tao Yuan Ming in which a fisherman passes through a cave opening and discovers a remote and paradisaical lost valley.

This sense of discovery is recreated by the approach to the museum, which is characterized by a carefully orchestrated sequence of concealment and unveiling. Visitors arrive at a reception pavilion set in a forest of cedars, a short distance from the museum. From here you either walk or take a small electric buggy through a gently curved tunnel, emerging from the bowels of the hillside on to a dramatic post-tension bridge cantilevering over a deep ravine. Framed by the network of splayed cables is the skyline of the museum beyond, a series of angular glass roofs and skylights rising ethereally above the luxuriant landscape. The configuration of the roof is, in fact, a dextrous elaboration on a simple geometric model. A trio of distinct glass roofs with reticulate triangular structures spring from massive boundary walls that rise and fall to follow the contours of the site. Each roof is oriented in a different direction in response to the topography. The roof form recalls the distinctive hipped profile of traditional irimoya thatched roofs, still prevalent in the region, and the corbelled eaves are reminiscent of Japanese temple architecture.

The bridge connects with a circular plaza at the foot of the museum complex. Terraced granite steps lead up from the plaza to the entrance hall, which extends laterally to form a glazed spine between two wings of galleries. Enclosed by an elegantly detailed steel space-frame, the spinal hall overlooks the undulating hills and valleys to the west. Light is filtered through finely slatted aluminium sunscreens and washes over walls of honey-coloured Magny Dore limestone. The surface of the aluminium slats is etched with digitized images to evoke the warm texture and grain of wood, and the rotated square profile of the screens reinterprets the vertical grilles found in traditional temples such as Horyuji in Nara.

Within the hall, the overwhelming impression is of boundaries dissolving between the natural and the man-made worlds. The long galleria-like space is filled with the presence of nature, like a huge scroll painting unrolled for contemplation. A group of ancient and weathered akamatsu pine trees frames the breathtaking view, in the tradition of shakkei or 'borrowed landscape', the Chinese and Japanese technique of creating a visual passage between an object in the foreground and one in the distance. Throughout the building, subtle framing devices contrive to make you constantly aware of the surrounding landscape, both in its raw and more formally manicured states. The latter is exemplified by the starkly abstract Japanese rock garden strategically placed in the square courtyard of the north wing.

Dug deep into the mountainside, the hermetic gallery volumes form a counterpoint to the visible, above ground spaces, as the museum slowly reveals itself in a sequence of transitions and shifting planes of light. At the north end of the horizontal glazed spine, a luminous stairhall leads to the Japanese galleries above. Cuboid alabaster lanterns, delicately veined like rice paper, are set into the monumental limestone stairs. Arranged around the rock garden courtyard, the north wing houses the museum's collection of Japanese art, with conservation laboratories and art storage facilities on the floors below. The south wing is devoted to the art of the Silk Route, focusing on the emergence and expansion of the great Middle and Far Eastern civilizations. At the south end of the spine are the Egyptian galleries with Near Eastern, Chinese, Islamic and Asian exhibition areas below. The south wing also contains a handsome auditorium, a double-height tea room with towering bamboo plants and various offices and meeting rooms for the museum's administration. At the lowest level, curatorial offices open on to a recessed garden, enclosed by a curved wall and planted with shrubs, pines and Japanese maples.

Each gallery wing comprises 1000 sq m of exhibition space and it is envisaged that the museum's collection of around 1000 objects will be displayed on a rotating basis. Certain galleries are also individually designed for specific works of art, such as the South Asia gallery, which contains a rare Gandhara Buddha dating from the second century. The design of the interior is intended to evoke the cave in northern India where the sculpture was originally discovered. The Islamic gallery, the tallest volume in the museum, was specially designed to accommodate an elaborately decorated sixteenth-century carpet, more than 6m high.

The Miho Museum marks a departure from I. M. Pei's usual design process of progressively elaborating on a controlling geometric strategy. Here, the rigorous constraints of the site have resulted in a looser, more flexible plan in which interconnected underground spaces spin off from a main spinal core. As project architect Tim Culbert notes, the notion of an 'improvised lyric' (used by Bruno Taut to describe the Katsura Imperial Villa in Kyoto) is perhaps a more accurate reflection of Pei's approach at Shigaraki, in which museum and landscape are fluidly and poetically integrated in a remarkable, unified whole.