Once upon a time, it was scary to think about opening day for the immense new Washington Convention Center. At heart, convention centers are just warehouses for the temporary storage of people and goods, and who wants an overgrown warehouse in the middle of town? Yet with opening night at last upon us -- an intimate group of 3,000 will gather in the center's spacious ballroom for this evening's gala -- it's happily clear that this worry was, in large part, unfounded. Yes, the center is the biggest building in Washington. It takes up the better part of six city blocks, stretching north from Mount Vernon Square all the way to N Street between Seventh and Ninth streets NW, a distance of close to five football fields set end to end. It rises 130 feet from the ground at its tallest point, higher than all but a few buildings in our height-limited city. It's a behemoth. But thanks to good architecture and an enormous hole in the ground, this behemoth has been tamed and, even better, groomed into polite, crisply presentable shape. In places, it's even thrilling. Before getting to those places, however, let's stick to the basics. Let's begin, that is, with the hole in the ground. In the mid-1990s, when battles were raging over the center's location, the powers-that-be realized that the only way to win their point on the downtown site would be to reduce the building's apparent size. And the only way to do this would be to dig a mighty ditch so as to bury a 500,000-square-foot room -- the center's main exhibition hall. This they did, and it was a wise, if expensive, decision. Putting the big room in the basement gave architects the chance they absolutely needed to link building with city in appealing symbiosis. The architectural team in charge of the project -- Thompson, Ventulett, Stainback & Associates of Atlanta (TVS), with the Washington firms of Theodore R. Mariani and Devrouax & Purnell Architects -- seized the opportunity. Rather than closing L and M streets, for instance, the architects were able to run the streets right through the building. Maintaining these roadways, closed during construction, has important urbanistic and architectural advantages -- easing crosstown traffic, permitting nearby residents to walk back and forth from Seventh to Ninth streets, and opening the building visually. Furthermore, burying the big hall gave the designers a realistic shot at lessening the visual impact of the building's massive size. This they proceeded skillfully to do with a varied series of elevations along Seventh and Ninth streets. Adroit changes in heights, shapes and window patterns make it appear almost as if there were sequences of medium-size buildings along these streets -- a transparent fiction, of course, but immensely preferable to the bulky alternative. But if the side facades were designed in the main to be rather low- impact affairs, the front of the building was conceived as a tour de force -- it's 90 feet tall and about 500 feet across, and the subtly concave central section, framed by limestone panels, is almost all glass. This isn't one of those drop-dead designs that you can't get out of your mind. It's a little too conscientious for that. But it comes pretty close, especially at night when the lights within make it glow like a giant, city-scale lantern. And, conscientiously, it serves as a backdrop for the Carnegie Library on Mount Vernon Square, one of those ornate, Beaux-Arts jewels that were the pride of many a U.S. city early in the 20th century. (The library, by the way, is soon to become the City Museum of Washington.) There was much concern that the big building would overpower the little one, but actually the reverse is true. In scale, the two work well together, the glass rectangle forming a splendid setting for the fancy marble box. If the building's exterior proportions are, by necessity, horizontal, those of the huge vestibule behind the front facade are surprisingly vertical. With its nicely modulated interior wall of a rich African wood, its tapering grand stairwell, its high pedestrian bridges and its exterior walls of glass, this is the center's architectural thrill -- a towering, light-filled civic room unlike any other in Washington. Of course, the functional heart of this building remains the vast underground chamber -- the belly of the beast, where much of the business of conventioneering will take place. Another exhibition hall -- only half as large but still very big -- and the 52,000-square-foot ballroom are stacked atop the building. The exhibition areas are like the outer layers of a sandwich, and the filling in between consists of meeting rooms, offices, kitchens and other support services. The special quality of this design derives largely from the fact that, in the main, these enclosed facilities are surrounded by corridors for trucks and people, and the corridors are sheathed largely in glass -- transparent for the people, translucent for the trucks. The strategy is characteristic of a new generation of U.S. convention centers -- the idea is to bring the outside inside, where possible, and to capitalize on the activity conventions generate to invigorate the design. This makes so much good sense that you wonder why the architects of Washington's "old" convention center -- a somber mediocrity constructed but 20 years ago -- did not think of it. But that was then, and now is now. In the new center, the architects made sure that there are a variety of definable places along the various routes -- well-lit, roomlike spaces on the 1,400-foot sunken concourse bordering the main exhibition hall, splendid city views in two directions from the bridge over L Street and wonderful glass-walled rooms atop the cantilevered "wings" that frame the main facade. Conventional wisdom in the convention industry has it that all the exhibition space should be on one floor. That's why, in Chicago and other places where there is plenty of room for expansion, they simply put up another new building when they need it. This was impossible in Washington and, not incidentally, it was all for the better: In several places the architects were able notably to dramatize the need for getting people up and down. On a busy day, the circular space and lengthy escalators at L and Ninth will offer a dizzying urban spectacle. Putting the big expo hall underground was a great thing to do, for sure, but it follows that lots of other stuff had to go underground too -- and underground, as a rule, is not a great place to be. The architects did wonders, however, getting lots of natural light into the burrowed concourse. But there wasn't much to do about the center's main restaurant -- it's a cave, and no amount of zippy interior decor will disguise that fact. (It's due to open in a few months.) While we're on the subject of bothersome things, let's talk about trucks. With conventions come 18-wheelers, and the center has docking spaces for 70. Simply getting the loading docks and ramps into the building without ruining the architecture was a design feat. (The trucks will enter and exit the building from M Street). Still, the fact remains that such trucks and residential neighborhoods hardly constitute a match made in Heaven. It'll take a lot of skilled scheduling and management to mitigate the undeniable negative effects. Likewise, it'll take both skill and goodwill to keep the center truly open in these insecure times, or to make anything at all out of the street-facing retail spaces. There are four of them, spaced widely apart, and they may turn out to be no more than symbols of good intentions. There are few things less enticing than stores with closed doors, but that's what is likely to happen here unless somebody mandates they remain open. If that means subsidy by the convention center, so be it. Then, there remains the issue of expansion. The worst thing about the location is that the center is locked in on all four sides. Some in the convention center industry talk about digging another exhibition hall ditch at the old convention center site, but they ought to give it up. It's a bad idea for conventions -- who is going to walk through a 500- foot-long tunnel to get to another part of the show? And it's a terrible constraint on the old convention center site, where the city hopes to create "a uniquely Washingtonian center for downtown urban life." So, we now have our new convention center and hopefully we'll have it for more than 20 years. Architecturally, the new is a tremendous improvement on the old, but of course that's not saying much. Let's put it this way: This is resolutely modern architecture with civic presence and contextual conscience. Great architecture it may not be, but it is a very, very good job with a tough building type, and that's not bad.