Philadelphia has not been lucky with parking garages. Architects are always promising to dress up these concrete monsters so they're indistinguishable from the other buildings on the city's streets. Has anyone ever been fooled?

Given the city's poor track record with its above-ground garages, I was dreading the new one at the Philadelphia Museum of Art. It's one thing to plop some raw concrete decks amid the hard surfaces of Center City. It's quite another to try to slip one in unnoticed at the epicenter of Fairmount Park's popular Waterworks gardens, a beloved and beautiful spot that is as iconic as any in Philadelphia.

As might be expected, the new garage hasn't improved the scenery. But the nice surprise is that it hasn't made it worse. For that, we owe a debt of gratitude to the designers at the Olin Studio, Atkin Olshin Schade, and CVM Engineers.

The 442-car garage is by no means invisible. Rather, the design's strength comes from its artful camouflage, including an elegant sculpture garden. While we can never forget the $32 million structure's presence, at least we don't have the ugly bits - the decks and ramps - staring us in the face. And because the structure is passively ventilated, we won't have to listen to the hum of fans.

As you approach from Kelly Drive, the garage doesn't immediately register as a garage because the designers tucked the structure into the sloping topography on the museum's river side, between the bronze parade of Revolutionary War generals, known as the Reilly Memorial, and the storybook setting of the Azalea Garden.

Cascades of boulders soften the exposed walls and help connect the intruder to the older landscape around the Waterworks complex. These same stones originally formed the rocky hillside, and probably were excavated two centuries ago for the creation of the Waterworks' reservoir, which now serves as the foundation for the Art Museum. We often forget that the landscape behind the museum is a constructed one. The garage continues the manipulation.

Still, that hillside had become part of Philadelphia's internal geography. It was a favorite spot for children to sled and climb. While the garage renders those activities impossible, the museum makes up for the loss by giving the city a lovely sculpture garden on the garage's roof.

Like the boulders, the sculpture garden is part of the camouflage strategy devised by Olin's landscape architects. They like to say they stretched the landscape over the top of the garage. The garden extends the museum's precinct into the lushness of Fairmount Park with a picturesque arrangement of winding paths, grassy moguls, and terraced overlooks.

There isn't much sculpture to look at - although more pieces are supposed to be coming - but the 360-degree views of the city, the park, and the Schuylkill are riveting enough. There are moments when you feel immersed in the primal wilderness of the Schuylkill Valley. Indeed, as I was admiring the thick tree line to the north, a red-tailed hawk almost as big as a turkey swooped down to size me up as a possible lunch option.

Yet, if you adjust your position a couple of degrees, the prospect shifts from nature's sublimity to the material world. The sweeping southwestern view encompasses a layer cake of industrial forces that made Philadelphia into a 19th-century power: the Waterworks, the river, its mighty dam, the railroads. The final topping is a ridge of red-brick Victorian houses, made possible by that industrialization. Turn a few more degrees and the Art Museum's golden-hued Parthenon looms above, representing the pinnacle of the city's cultural aspirations.

Together, the sculpture garden and its two terraces sprawl across a full acre, making the space the city's largest green roof. The terraces act as distinct rooms and step down to the level of the Italian Fountain. A formal glade of trees offers some modest shade on the upper terrace, while the lower enclosure is bracketed by curtains of water that emerge from the walls, mimicking the falls at the river below.

It's too bad the plantings are a bit thin, though they will fill out. More jarring is the black asphalt used for the paths. While it will fade, the softness of the sculpture area would have been better matched by fine gravel, which was used only on the upper terrace.

Like many Olin landscapes, the detailing is so restrained and subtle that the place hardly seems designed at all. A glass safety fence rims the garden and terraces. Much like the garage itself, the fence isn't invisible. It's simply designed to call as little attention to itself as possible, and to serve as a neutral backdrop for sculpture.

The best of Olin's designs, like the recent remake of New York's Columbus Circle, have a logic that makes them appear inevitable, as if there was no other way to approach the problem. The sculpture garden lacks the circle's rich detailing, but its organization does put it in that category.

Unfortunately, the same can't be said about the garden's most prominent element: the glass elevator house. The architects at Atkin Olshin Schade were obviously following the strategy that informed the rest of the garage, designing the cube to go unnoticed. But you can't avoid seeing it, and it's just as heavy-handed as the elevator house on every other garage.

Claims of invisibility for glass structures have become an architectural cliche. Even the Apple cube in New York, which is as unencumbered as they come, remains a visual presence - albeit a thrilling one.

Given the elevator house's prominent location in the museum's sculpture garden, the structure cried out for a more imaginative treatment. Invisibility can be taken too far. Where was Frank Gehry - who is designing the museum's new underground galleries - when Philadelphia needed him?

Contact architecture critic Inga Saffron at 215-854-2213 or isaffron@phillynews.com.