Built on the site of Altamira's prehistoric caves, Juan Navarro Baldeweg's new museum has a poetic affinity with the surrounding landscape. Within this subterranean realm is a diverse set of spaces for exhibition and research.

The museum constructed by Juan Navarro Baldeweg on a hillside in Cantabria is somewhat paradoxical. This container houses a replica of the famous Altamira Caves: the underground site of prehistoric art is nearby but -- for essential preservation reasons -- no longer accessible to cameratoting, vapour-breathing tourists. The new building protrudes only slightly above ground level, follows the slope of the land, and is coloured or textured using a palette that is almost like environmental camouflage. Yet Baldeweg's latest addition to Spanish architectural culture is neither subservient to its context nor apologetic of its true nature and intent. It is elegant and bold within reason.

The design, which dates back to 1994, seems to evolve around sets of dualities. There is, first of all, the authentic cave now secluded from view and its facsimile double, a situation that in lesser hands might have led to second-hand kitsch and a ridiculously inferior experience. There is the duality in section between the (second) cave hollowed out of the ground and the expansive dome of the sky high above. And there is the duality in plan between the column-free box sheltering the surrogate cave and the immediately adjacent exhibition wing arranged as a perpendicular cascade of interior terraces. This other half is not unlike one side of a Baroque garden parti. I visited the caves, as a teenager in the 1970s. The former visitors' centre and car park seem smaller when revisited a quarter of a century later. The new museum is further on along the contours, further away from the road leading from the picturesque rural town of Santillana del Mar (a distance also less than that remembered). Baldeweg is a nativ e of this temperate province between the Pyrenees and Galicia. Although based in Madrid, he retains an indigenous respect for topography and climate, an ease also manifest in his early and well-named House in the Rain in the hills behind Santander. The architect's own memory and knowledge have undoubtedly assisted Altamira.

Approaching from the east, you first glimpse the slanting parapet, its ochre or dull gold-coloured aluminium siding glinting in the morning light. Then, as the extended range of the building presents itself to the north, the great sloping roof is revealed as a series of striations, an artificial geology Including linear light monitors and strips of grassed turf. Walls of local ashlar to the left and a flush stone dado to the right augment this feeling of sedimentation, of Baldeweg's building being literally of the earth. The dado looks as if each panel of stone might, like some prehistoric machine, have been simply tilted up in place.

Crudely put, the museum is a shed. Having driven past to where cars and buses are parked in tray-like extrusions out into the fields, visitors approach the building again from the north-west. Another wall of honey-coloured ashlar comes forward with a further layer behind of a pergola and cafe terrace. Cows can be seen and heard on the meadows above, towards the south. The museum wing with its low light monitors (reminiscent, perhaps, of certain instamatic cameras) rises towards the deep green farmland, the monitors' upright north faces made of cool frameless glass.

Entry takes place in the terraced section at the point where it and the sheltered cave diverge -- the latter seems to shear subtly to the left and into the hillside. The entrance, identified by a sharp flat canopy, has its flanks painted a telluric red. To the left, an orthogonal void cut like a proscenium or picture frame is clearly a signal of something important deep in the interior. From the entry, internal trays ascend to the right, each with a single horizontal band of ceiling tipped to allow north light in. Just inside the north elevation, a strip of floor used for lectures and meetings is lowered so that you find yourself momentarily below ground.

Access to the facsimile cave is by a sequence of internal stairs slotted into the seam between the project's two principal parts. From down there, buried one storey deep in the earth, you look up and see the sky framed by the single aperture, a glimpse positing today's tourist in a relationship with the outside similar to that of the cave's original visitors. Baldeweg's work is in general noticeable for its investigation of primary volumes (cubes, cylinders, hemispheres) -- his library at Madrid's Puerto de Toledo, for instance, or the Conference Centre in Salamanca (ARs December 1994 and July 1990). Being unusually contingent upon context and content, Altamira is also a resonant reflection of Baldeweg's other interests as a painter. There is, undoubtedly, a painterly quality about the yellow aluminium with two finishes of stone (vertical surfaces), and the tilted grey aluminium strips --the light monitors' southerly face -- inserted amid striations of green grass (horizontal canvas or roofscape). Emerging f rom this latter surface, the artificial meadow, are of course the light monitors. Baldeweg is interested in colour and aesthetic composition but also in the devices that illuminate his interior world. Considering the other half of the building, especially the rectangular void in front of 'the cave', we realize perhaps that the architect is primarily concerned with mechanisms of seeing.