A light in the earth. Shafts and pools of light illuminate art of Africa, Asia, and the Near East in Washington's two new subterranean museums

`SO here we go down ..., down 64 feet...,'' says Jean Paul Carlhian, the architect who is leading us deep into the earth and down to the light - down towards his two astonishing new underground museums that plunge far below the level of their neighbor, the Smithsonian Castle. Like Alice disappearing into the rabbit hole or Orpheus descending, Carlhian leads us below to the wells of natural light that illuminate the new museums: the National Museum of African Art and the Arthur M. Sackler Gallery for Asian and Near Eastern Art. The $73.2 million, three-story, below-ground complex, which also houses the Smithsonian's International Center, will open Sept. 28. (Funding came from $36.6 million in federal money, and from private donations.)

``You go down to hell; you go down to Filene's Basement...,'' says the debonair French architect from the Boston firm of Shepley, Bulfinch, Richardson, and Abbott. ``But you go up to heaven - you go up to the altar,'' he adds, referring to the treasures that will be contained below.

``There I was, asked to take people down to multimillion-dollar collections. Therefore, I had this preoccupation with how to make their journey down as exciting or adventurous or enticing as possible, ... elevating their spirits as they were going down.''

He decided to lift their spirits by having natural light penetrate the underground rooms. There are huge, transparent rose windows and vast skylights that create unexpected shafts and pools of light at every turn. The pale limestone used in both museums was chosen for its reflective properties. On the third level down, which is below the water table of the adjacent Tiber Creek, there is even a small round pond of light paved with tile imported from China and filled with water that reflects the sky far above. ``We always try to bring light down,'' says the Paris-born architect, who is a graduate of the Ecole des Beaux Arts and Harvard University.

That was his vision as architect when S. Dillon Ripley, then secretary of the Smithsonian, hired the Boston firm to carry out his concept of double museums embracing the art of half the world. The original design concept for Ripley's underground twins was done by Junzo Yoshimura of Tokyo.

The vastness of the project Carlhian has worked on for eight years is measured in terms of an excavation ``big enough to hold three Lincoln Memorials side by side,'' the architect notes. ``The earth taken out [371,000 tons] is equivalent to two Queen Elizabeth II ocean liners. It was a very big hole.''

It was also a very big challenge: Create two underground museums, each with its own above-ground entrance pavilion, but design them in styles that won't battle with the romantic, red-sandstone Smithsonian Castle or the Arts and Industries Building (A&I), both designed in high Victorian style by the celebrated James Renwick Jr.) or with the neoclassic Renaissance design of the Freer Gallery. And make it harmonize with an elaborate triple garden covering 4.2 acres - a Victorian ``Parterre'' central garden, flanked on the east by one inspired by the Alhambra, with its African/Islamic culture, and on the west by one with an Oriental flavor, hinting at Peking's Temple of the Sun.

When Carlhian's firm was asked to design the ``quadrangle'' buildings in front of the Castle, revisions in the Yoshimura concept were found to be needed. Carlhian says of the entrance pavilions: ``I could have [had] a baby Freer next to the Freer; I could have [had] a baby A&I next to the A&I, right? And if I did that, the romantic [Victorian] would still fight the classicist. [Or] I could say, `I'm going to be an architect and do my thing.' But doing my thing would increase the confusion. There are already two Victorian buildings and a Florentine palazzo, right?''

His solution was to crossfertilize the architectural motifs between the buildings. He repeats an arch or arcade motif from the Freer in the domed roofs of the African pavilion across the garden. And the pyramid roof line of the Victorian A&I is repeated in the pyramid-shaped roof of the Sackler pavilion. The pyramid theme is also echoed in diamond-paned windows. A third building, a brioche-shaped kiosk at the northwest corner of the quadrangle, leads to the Education and Conference Center three floors below.

Since 96 percent of the museum structure is underground, the two small pavilions and kiosk are like architectural periscopes for the submerged treasures. Carlhian says he tried to make the pavilions serve as ``frames'' for the Castle. In front of the Castle, in the Victorian garden, he handsomely masked two required exit staircases with the Castle's red sandstone. At the suggestion of National Gallery director J.Carter Brown, they were designed to look like romantic tombs for Romeo and Juliet, with pilasters, cornices and copper roofs.

Other challenges were involved. Carlhian had to work with seven city groups, from the Fine Arts Commission to Sierra Club. But he says he found most of their suggestions constructive.

There was the construction challenge of doing the massive excavation without, upsetting the fragile foundations of the Castle and other buildings. He also had to notch the new building to make room for the roots of a 100-year-old Linden tree that Ripley wanted saved; protect the building from the Tiber Creek waterline with an elaborate pumping system; and design a museum roof strong enough to support the four feet of earth above it, on which the gardens grow.

``It was a tricky thing,'' Carlhian says of an excavating technique using a ``slurry wall,'' a new process in which a pudding of bentonite clay is poured into a trench around the excavation site. The slurry, which has the same density as the soil, prevents shifts in the foundations of nearby buildings. When the slurry is pumped out, concrete is pumped in, in this case forming the museum's foundation walls.

As Carlhian winds up a two-hour tour of the quadrangle, our feet crunch across the pinkish-gray gravel of the path leading through the triple garden. We pass the modern ``moongates'' of red, sand-blasted Texas granite; the chadar (waterfall) inspired by the gardens of Shalimar in India; and the stunning wrought-iron entrance gate, made to order from an original design Ripley found by Castle architect Renwick.

At the end of the long tour on a 102-degree day, we sit in the shade of a tree and watch a little girl with a blonde pony tail approach one of the jets that spurt water from the middle of granite benches. ``Take your shoes off, and wash your feet!'' Carlhian calls to her. ``It was made for that.'' She does and squeals with laughter. ``That's exactly the idea. I did it purposely,'' says Carlhian with a satisfied smile.