Modern architects have a long tradition of trying to reimagine the relationship between buildings and nature. Philip Johnson famously designed the walls of his Glass House so it appears that only the barest membrane separates its occupants from the rolling Connecticut countryside. At Fallingwater, Frank Lloyd Wright envisioned the house's cantilevered terraces growing organically from the boulders over the thundering Bear Run torrent.

New York architects Marion Weiss and Michael Manfredi take an entirely different path with their Krishna P. Singh Center for Nanotechnology at the University of Pennsylvania. Instead of focusing on the boundaries between architecture and landscape, they fuse the two into a seamless whole. Their dazzling new project is like the mythical centaur; you can't exactly say whether it is man or beast.

So, yes, what is clearly a large glass building now occupies a former Penn parking lot on Walnut Street, just east of 33d Street, enlivening the forlorn eastern edge of the university. But the way the Singh Center emerges from the ground and ascends in a series of crystalline switchbacks suggests the structure might be some long-buried geological formation that just happened to erupt in the middle of West Philadelphia.

Call it a man-made landscape if you want, or call it topographical architecture, but the $92 million Singh Center is easily the most impressive new design in the city since the Barnes Foundation opened last year.

A lot of dense architectural theory went into its shape, which is interesting to the cognoscenti. But the interior spaces, as blissed-out as a mountain retreat, are what will win over the nanotech researchers, who started moving in this month. Modern science is as much about socializing as peering into microscopes, so there are many spots for exchanging ideas, from diner-like booths to nightclub-like lounges, done up in pops of paprika and marigold.

The most distinctive feature of Weiss/Manfredi's shimmering outcrop is a daring, cantilevered glass box that juts out and over the building's grass forecourt. You can't look at the precarious, 68-foot-long extrusion and not think of the cantilevered terraces at Fallingwater, which the husband-and-wife architects consider a touchstone for their work.

But instead of hovering over a rushing mountain stream in Western Pennsylvania, the Singh's cantilever floats 40 feet above the rushing flow of students along Walnut Street coming and going from Penn's campus. Inside the glass box, there is a small auditorium and the modernist version of a sunporch. I imagine a huge competition for the porch's custom-designed, paprika-colored armchairs, positioned at the precipice of the cantilever's window wall. From there, you can survey nearly all of Penn's domains.

For all the architectural spectacle on display, the Singh is essentially a souped-up lab building for the nanotechnology department at the School of Engineering. Nanotechnology is the rapidly expanding field in which scientists manipulate molecule-size particles to devise new products and medical treatments. The close work requires electron microscopes and other delicate machines that are sensitive to vibrations and ultraviolet light.

As one of the first Penn buildings people encounter from Center City, the Singh was envisioned as a campus gateway, greeting students after their trudge across the Walnut Street Bridge. But because of the need to insulate lab equipment from bone-rattling traffic, much of the L-shaped building had to be set back significantly from the sidewalk.

That took its toll on the idea of a welcoming portal. What visitors see instead are the rippling metal panels of the building's side wall. Large windows on the upper floors supply a beacon of light at night, but the effect is not the same as being embraced by the sight of human activity on the ground.

Still, putting the Singh on the site does have the benefit of filling in what had long been a missing tooth in Walnut Street's continuity. More than that, Weiss/Manfredi, who once worked for renowned Philadelphia architect Romaldo Giurgola, cannily use the rhythms of the wall's vertical panels to create a dialogue with the adjacent parking garage - designed by Giurgola in 1963. It may be hard to believe, but its immense, diamond-shaped concrete trusses were once an architectural sensation, and even now exhibit a primal, sinewy beauty.

It is a testament to the talents of Weiss/Manfredi, who designed the Singh in collaboration with the M&W Group's lab specialists and Severud Associate's structural engineers, that the setback does not detract from the building's real quality.

Like the architects' recently completed visitor center at the Brooklyn Botanical Garden and their well-known Olympic Sculpture Park in Seattle, the Singh is a topographical structure in an urban context. The forecourt, tucked in the crook of the L, is its own landscape: A sculpted hill and a wedge of grass serve as foothills for the building.

The lawn ramps up to the entrance, drawing in visitors and the eye. Then it ramps down into the earth, creating a wide crevice that allows light into the basement level. At the building entrance, a repurposed Tony Smith sculpture, We Lost, stands sentry. The spare black Mobius strip is a thematically perfect counterpoint to the building's white switchbacks.

Once inside, visitors encounter an unusual paprika-colored glass wall. Chosen because the color blocks ultraviolet light into the labs, it set the palette for the entire building, which plays the paprika-marigold-orange accents off the white finishes.

The Singh is the last of a trio of notable building designs masterminded by Eduardo Glandt, the long-serving dean of Penn's engineering school, with the intention of luring top researchers. For $92 million, including a $20 million naming gift from Krishna Singh (one of the owners of Interstate General Media, parent company of The Inquirer), Glandt obviously could have packed a lot more labs on the site.

Instead, Glandt chose to invest in something less tangible but potentially more valuable: architectural magic. Just as nanotechnology enables researchers to see a miniature world never before glimpsed, the Singh's design promises surroundings to help open their eyes to new ideas.