Primal therapy.

Resonant with an elemental materiality and full of myriad therapeutic delights, Peter Zumthor's thermal baths at Vals are conceived as a cavernous, labyrinthine haven devoted to sensual pleasure.

Vals lies an hour away from Chur by car, deep in a valley dotted with shepherd huts and enlivened by the sound of cowbells. Above the village, a tributary cascades down to meet the upper Rhine. It's a peripheral spot, dependent upon agriculture and tourism. A century ago, hot natural springs were first diverted for therapeutic bathing and in the early '60s a vaguely glamorous hotel was erected to profit more intensely from the spa. However, as a result of financial failure in the 1980s, the small municipality of Vals took over the business and initiated an architectural competition to reestablish thermal bathing as an attraction for a wider clientele. The winner was Peter Zumthor, who has as usual worked in intense contact with the project and its site.

From above, the new building is almost invisible. The hillside meadow slopes down to spread horizontally out onto a terrace which will soon read as a carpet of blue flowers. In this field are fissures of translucent glass and a square bed of downlighters, a little like mechanical sunflowers. The roof is protected from the 'meagre meadow', and from the hotel complex to the north, by a simple railing but then erodes towards the south to reveal a swimming pool and sunbathers on flat slabs of rock. From the road below, the building appears as an embankment, a monolith of compressed stone with large ocular openings. Not so much a building as an earthwork, Zumthor's design is about digging and mounding up; it's archaic and primary. It's also extremely sensuous.

Access to the Baths is along a curving tunnel from the hotel. The subterranean nature of this connection is crucial as it dislocates the individual from the world outside. There follows a knight's move through 90 degrees to clear a tubular turnstile and through another 90 degrees again to align yourself with a long, shadowy corridor from which you can hear the trickle of several faucets. This brings you to the upper level of a tiered section. To the left, a gap offers a peripheral glimpse down onto the main internal pool and out one of the big openings to the valley below. The wall to the right is homogeneous concrete, indented only with ' some square fountain heads (dripping into a continuous gap between wall and floor). The vertical surface towards the pool becomes a flank of horizontally-laid stone broken in five identical places. These are the changing booths, screened in curtains of black leather.

Behind these drapes, each volume for undressing has flush walls of lockers and a single leather banquette. Diverging from Zumthor's basic palette of concrete and stone, they are panelled in highly polished red mahogany, exquisite cabins waiting to be touched by the bathers' naked skin. Stepping out, you find yourself standing on a terrace above the principal indoor pool. There is a wing off to the right (containing showers and lavatories and, beyond that, steam rooms), but attention is focused ahead onto the surface of the water, at the play of light, and the slowly descending stepped ramp down which every able body must proceed. The ramp is clearly ceremonial, slowing down even the most ardent bather in a ritual of shifting geometries. A linear gap in the roof above admits a bright strip of daylight. Then, across the lower stone floor, you descend - again, slowly - into the warm navel-high waters of the main pool.

This is a buried, almost labyrinthine world of solid and void within which the spa water is retained. The main pool is a rotational space, to which the bather always returns. All around are massive stone shafts with streaks of sunlight from above and vertical planes of light beyond. Directly above are 16 small bright blue rooflights, the underside of the light fittings in the 'meagre meadow'. If the big move at Vals is to reform the hillside so that the Alpine topography becomes inhabited (part-cave, part-viewing pavilion), Zumthor's next tactic is to make generous interlocked spaces from these pinwheeling blocks of stone. The blocks, which are in turn revealed to contain small orthogonal rooms, might be thought of as having been carved from the mountain, but are built up by the architect as volumetric structure.

Although devoted to the truth of materials, Zumthor is rather coy in his explanations of structure. The Baths are in fact a composite of in situ concrete and load-bearing gneiss from a local quarry. None of the stacked stone is, in Zumthor's world, insulted by being merely applique. Seen from the hillside, the mass of his building is split by thin fissures of glass, so that the entire form seems to break into geological outcrops. From within the Baths, the fissures (topped with several layers of glass) mark at least one edge of each stone shaft about the central pool. This results in certain flanks being washed in zenithal light, but also divides up the ceiling plan so that each shaft, housing supplementary structure, supports its portion of roof. Like structural lily pads, these concrete slabs interlock as a canopy above a floor, itself composed of rectangular panels of stone.

The gaps between these lower panels form thresholds and channels for excess water. They delineate the inhabited shafts - each with its tiny specific chamber- from the general pool precinct. Inside one apparently solid shaft is a chilly 10 degrees Celsius plunge pool, inside another an aromatic 30 degrees C bath with petals; both are entered at right angles and surround the bather immediately in stone. You step down directly into the hottest pool (42 degrees C), then rest on submerged shelves as small waves drop noisily into a deep perimeter trough. The 35 degrees C pool is beneath the point of entry, but turns back through a small chasm to reposition the more adventurous bather in a high chamber lit from below. Across the plan, another body of water moves out against a tall external window; in summer, the lower panel falls away to allow swimmers direct connection into the big outdoor pool.

The gneiss is meticulously laid in bands of varying depths with visually neutral mortar. Different levels of polished smoothnesses bring out the sparkle of the constituent mica and quartz in the stone. Around the main pools, where illumination comes from above, the stone below water level appears dark and viscous, pale and desiccated above. But in the pools lit from below, the opposite occurs. Hot and cold rooms are lined in terrazzo (terracotta pink and baby blue respectively) so that the bathers' attention is initially focused on the water and only subsequently on the surrounding surface, A third chamber, with drinking fountain and a mysterious well, is so dimly lit that its sides of smooth flags are barely perceptible. A fourth - the darkest, or least reflective - is a kind of introverted speaker with leather beds and a body-activated loop of music composed literally by playing with stones.

If Zumthor's section is a fixed one-water, after all, finds a single level- his plan manoeuvres to find a myriad possibilities. Visitors certainly seem to enjoy discovering and experiencing the Baths' nooks and crannies. As the principal floor extends around the outdoor pool with its three jets of water, the stone rises to form flat decks for lounging in the mountain air. Small cabins help screen this rather sybaritic scene from the village below. All these blocks along the high easterly elevation contain massage or relaxation rooms (there are more, beneath, for mud treatments and physiotherapy). Their square windows, within slim steel boxes, sit at chaise longue height for comfortable viewing. Although never mechanical or institutional, the Baths retain a clinical aspect.

There is one further material. Doors, handrails, grips, suspended and applied signage, the discs of pendant lights, and the tubular contraptions at entry and about the drinking well are all made of bronze. Even the sipping cups and their attachment chains are made of this wonderful substance, Its occasional roundness and dull metallic glow contrast splendidly with the grey homogeneity of the cavernous setting. The bronze of the linear balustrade and handrails interacts with the slits of descending natural light; doors meld into their openings as immaculate frameless surfaces.

Through the rigour of his craft, Peter Zumthor has realised an extraordinary building full of sensory richness. From the earlier projects in Chur, and from the church of Sogn Benedetg west of Vals (AR January 1991), Zumthor has now developed an architecture of complex spatial interpenetration. At Vals, he has created a building concerned not simply with style, image or beautiful materiality, but resonant with atavistic memories of weight, contiguity and enclosure, of sound and enticing illumination. To use the Baths is an intense, almost primal pleasure.