One must first step back and admire the awesome pluck of it all, the complexity and almost loony flair of the idea: There's a delightful park, a noble tree, three new and pretty buildings and then, under these, galleries, loading docks, storage rooms, libraries and offices for two major museums, plus a brand-new international institution, an auditorium, classrooms and enough additional administrative space to fill, in the world of real light and real air, a modest-sized office building.

This is, of course, the Smithsonian Institution's new Quadrangle on Independence Avenue south of the Castle on the Mall, comprising the Arthur M. Sackler Gallery, the National Museum of African Art and the S. Dillon Ripley Center, each opening tomorrow, and the Enid A. Haupt Garden, the Smithsonian Associates (Resident and National), the Smithsonian Traveling Exhibition Service, the linden tree and so on.

It is a project of a scope that is hard to comprehend, a wildly ambitious puzzle put together in the fertile, not to say febrile, brain of former secretary-impresario Ripley, who, as much as anything else in his 20 years in the post, got things built. To come to terms with the feat it is necessary to try to see it whole, as Ripley did -- no easy task.

When it was just an idea, there were lots of negatives. No one thought the Congress would sit still for, much less enthusiastically support to the tune of $36 million (about half of the actual construction budget), another building on the Mall. Though many agreed that the Smithsonian's adoption, in 1979, of Warren Robbins' Museum of African Art was a good and needed action, few foresaw the opportunity actually to build a new physical plant for it. And few could figure out how to get around the curious stumbling blocks Charles Lang Freer put in his will, prohibiting hisgallery full of Asian masterpieces forever from borrowing objects (or from lending them).

But Ripley, the inspired opportunist, treated these apparent obstacles as a lifetime chance to assemble what real estate developers would call a "critical mass." When he decided simply to put them all together and to bury the whole complex, there were predictable howls concerning the Victorian garden (which Ripley, by the way, caused to be built in the first place) alongside James Renwick's much-beloved sandstone Castle, as well as concern about the effects on the architectural and structural integrity of the Castle and Arts and Industries buildings. There was an audible collective gnashing of teeth throughout the Smithsonian about the fate of the linden tree. There were significant technical problems -- an underground river runs right through the site -- and there was understandable reluctance in the bureaucracy to moving appealing if scattered offices permanently underground. Nor, also understandably, did too many people gather close to heart the idea of exhibiting masterful works of art in underground chambers.

And Ripley's misty talk of topping the whole enterprise off or, literally as it developed, of bottoming it out with an international center where "nearly 140 nations will have a theater of action here to exhibit, to express their cultural histories and their creation of much of the roots of our own civilization" was received with respectful bafflement. He was, it was widely felt, exercising a conceptual sort of droit du seigneur in his final high-flying act as secretary. By then, of course, the whole strange, compelling package had attained an irresistible momentum.

So now we have it, this quadrangle, and the question naturally arises, what is it precisely that we have? Do the museums, and for that matter the other parts of the project, have strong individual identities above and/or below ground? Do they come together as a whole or do they still look and feel like the tag end of Ripley's cultural shopping list? How much is too much to shove underground? Do any, or many, of those initial negatives remain?

The first and, fortunately, in some ways foremost answer to architectural questions is that we have a splendid setting close by the Mall, a special new place tucked into a niche alongside the nation's preeminent public park. That the whole works cohesively above ground is crucial exactly because the bulk of the project -- 96 percent of it -- lies in wait underneath the soil. If the park and the buildings did not work so well together, there disastrously would be no there there, but fortunately the quad has a sense of identity that one can happily spend years warming up to.

The best way to get a sense of the whole, and to begin to understand the architectural issues involved, is to perch oneself for a while on the edge of one of those plantless planters in the long plaza fronting the Forrestal building directly across Independence Avenue. From here one commands a view of the entire site: on the left, the solid, understated clarity of architect Charles A. Platt's Freer Gallery (completed in 1922), based upon a Renaissance ideal; on the right, the sharp, airy, polychromatic Arts and Industries Building (1881), fancy but functional, designed by Adolph Cluss, Washington's premier public architect of the time, with Paul Schulze; and in the center, behind sandstone gates rebuilt according to Renwick's original design, his Castle (1849), rather solemn in spite of its vivid, romantic profile.

Framing the Castle, and adding considerably to the architectural interest of the composition, in fact making a true composition out of what was just a chance composite, are Jean Paul Carlhian's sturdy, but exquisite, pavilions for the Sackler and African museums. Carlhian, a French-born U.S. citizen trained at the Ecole des Beaux Arts in Paris, and design principal for Shepley, Bulfinch, Richardson and Abbott of Boston, understood the space and, as Japanese architect Junzo Yoshimura (the first designer on the scene) did not quite, the need to respond directly to the eclecticism of this ensemble. His achievement, in the exterior design of the pavilions, was to tie the pieces together, to unite beaux arts formality with picturesque romance in two beautifully proportioned, finely detailed buildings, and to do so without overwrought postmodernist gestures. This architecture is a little bland, there's no intellectual edge to it, but in compensation the buildings have an elegant, comfortable, permanent look.

The pavilions are located in just the right place and seem just the right size for the site. Their rooflines, a neat little troop of copper pyramids for the Sackler and of domes for the African museum, are ordered, graceful and picturesque -- they fit the unique surround. Their symmetrical stone fac ades, comprising a scheme of squares tilted on edge for the real and blind windows of the Sackler and of circles for those of the African building, are complementary. Their materials, grayish granite for the Sackler and reddish granite polished in varying degrees for the African, are appealingly tactile. The third principal building on the site (there also are a few somber little fire-stair enclosures) is the "kiosk," a folly with a scalloped helmet dome serving as entrance to the Ripley center, classrooms and Smithsonian offices.)

Around these buildings, in the Haupt garden, are arranged a variety of fetching places. There's a spare stone garden with offcenter moon-gate entrances, also made of granite, alongside the Sackler pavilion, and a stone-paved fountain with sitting ledges alongside the African. There are the benches circling in the shade of that linden tree -- a sizable notch was cut into the plan of the underground building to allow room for these old roots. Axial relationships are paid attention to. The Victorian parterre centers on the Castle; the fountain plaza on the African edge of the site brings the side door of Arts and Industries into focus. On the whole there exists a pleasing balance of informality and order here -- rather than face the cold, broad stretch of Independence Avenue, repeat visitors I'm sure will try to approach the museums from this unusual, and quite enchanting, outdoor place.

This is, as I said, all to the good, for the flip side of architectural rectitude greets one immediately upon entering either of the museum pavilions. The entrances themselves are problematic. They are elegantly made but so low key they're a bit hard to pick out from the overall symmetry of the fac ades -- a foretaste of circulation problems one will encounter throughout the underground complex. But it's what's inside the glass doors -- nothing -- that really startles. Well, "nothing" is too strong a word, and yet it's justified by the absence that I felt -- no number of plants and no amount of architectural manipulation will rescue these high, cold, stone-walled, neutral enclosures.

What is needed, and what one of course expects, is art -- an explosion of signification of the kind so carefully denied on the outside. It's almost enough to make one wish that Yoshimura's self-consciously "ethnic" designs had been built -- admittedly a bad idea (how does one say "Orient," how does one say "Africa" in a single building?). The Sackler pavilion, at least, has a few objects strategically located -- a stone Chinese Buddha head, a Tang Dynasty ceramic horse, two 2,000-year-old jars -- but this isn't nearly enough. Ripley's conception of two "grand vestibules" was flawed from the outset, and Carlhian's noble attempt to celebrate the art of architecture in these otherwise artless spaces didn't come off -- the geometry of those high domes and pyramids is asked to do too much. It's strong but not moving enough. One can imagine, for a discombobulating moment, that these are (a) flowerless botanical gardens or (b) wings of a new Smithsonian Metro station.

Carlhian's main effort, however, was to make a virtue out of a reversal of expectations: We're accustomed to going up, not down, to museums, and we're used to seeing art (at its best, anyway) under conditions of deflected natural light. He strove mightily -- it's a big problem -- and he succeeded in part: His descending stairwells are things of grace, vertical tunnels (a scalloped circle in the African pavilion, diamond-shaped in the Sackler) with well-formed skylights at the top and tiled pools of water at the bottom. The spaces are spellbinding at certain stops along the way, although on the lower two of the three underground levels they tend to dump the visitor unceremoniously upon platforms leading to closed doors. One's locational sense is easily unbalanced in the underground, and unfortunately though perhaps inevitably, the deeper these tunnels of light get the less they serve as focal points for orientation.

Getting lost in an art museum of course is not an unblessed frustration; indeed, it's a privilege, in a way, and one not confined to underground spaces. Certainly only a detestable grouch could complain about what one gets to see and to think about while wandering around in these depths -- the artwork is first rate and the exhibition design teams (headed by Richard Franklin for the African museum and by Patrick Sears for the Sackler) did extremely well by such rare and beautiful objects. Franklin's finely scaled installations are spare, abstract, and Sears' are contextual, imagistic, but long live the difference. The Sackler's public places, however, are homier, more welcoming -- one gets a view of a work of art at almost any turn.

Two big, though not necessarily good, ideas to relieve the underground gloom were discarded as the museums were built. Yoshimura proposed two sunken courtyards alongside the pavilions to bring light down; this idea was scuttled (and rightly so) for the greater good of the ground-level park. And Ripley and Carlhian envisioned a high-ceilinged room, a "great hall" linking the two museums underground, surrounded by hallways with overlooks. This was one of those notions that started heads nodding up and down -- yes, um-hum, yes -- and the room actually was constructed. But predictably the museums' need for wall space meant that the overlooks (with a sole exception in the African museum) would be covered over, and, also predictably, the two museums went their separate ways, each taking half of the space. The only connecting links are subtly disguised doorways that'll baffle tourists while regulars quickly learn to use them as shortcuts.

Underneath all this, in the basement of the basement, is the exposition hall of the Ripley Center, a big, longitudinal space currently occupied by "Generations," one of those only-at-the-Smithsonian multidisciplinary exhibitions with a wide world full of objects concerning the family of man. The variety of genuine objects in the show is fascinating, but here -- and it can't just be the effect of all of those blown-up baby pictures -- one begins to feel in dire earnest the need to escape to light.

Fortunately, if confusingly, there are three ways to go -- back up through the African or Sackler pavilions, or along a broad corridor that empties onto the underground "street" on the northern side of the site ... another world entirely. It is the world one enters from the kiosk, Carlhian's architectural "folly" (meaning, of course, not his caprice but a fanciful small building free-standing in a park), a curiosity destined to become a likable landmark. Its form, in any case, is not the problem; what's underneath it is.

The kiosk surrounds a circular stairwell, which leads to an escalator ride, which deposits the visitor on the underground pedestrian street opening on one side to the auditorium and classrooms, and on the other side to the Ripley Center and various other Smithsonian agencies. One can understand the architect's intentions here and appreciate his results -- the closed-to-open sequence of underground spaces is well conceived. But at this point the demands of the program overwhelm architectural ingenuity. In this underground warren the offices are definitively enclosed, and the dimly lit street, with aquarium-like "natural" light filtering through from four distant skylights, offers no true surcease to the surreal subterraneanness. Richard Haas' mural, at the long end of the corridor, imaging the ensemble as a classical ruin with a longing view up to the Castle and the sky, says it all.

There is, of course, a respite for cavern fever in that wonderful park with its grasses, trees and pretty pavilions, and though I'm no fan of underground art museums, the African and Sackler galleries prove that if the art is good enough it can be done and done well -- we can dip down from the quad on great staircases for an encounter with something inspiring. The very word "quadrangle" conjures up an image of a pretty, open space framed by buildings, a place where busy collegial interchange spontaneously occurs. But the image and the reality don't quite mesh here -- while one admires the scope and economy of Ripley's brilliant conception, one can't escape the consequences. The other side of such a genius for getting things built here proves to be building too much -- underground. As an urban, or even collegial, model, this is one that shouldn't be widely imitated. Then again, it's an epic fait accompli -- we'll all be going there often and, for that reason among others, can only wish it well.