Almost 25 years have passed since former Philadelphia Mayor Ed Rendell made a cold call to Alexander S. C. Rower, head of the Calder Foundation in New York, and offered to build a museum dedicated to the work of his grandfather, Alexander Calder, the innovative modernist sculptor who invented the mobile.

Improbably, Rendell and Rower reached a deal, even though other cities, including New York, had been vying for the project. Philadelphia officials quickly designated a site on the Benjamin Franklin Parkway for the new Calder museum. A famous architect was hired. Benefactors pledged money. Media events and celebrations followed.

Then, crickets. In 2005, the Calder museum was officially declared dead.

But now, just as improbably, the project is being resurrected by philanthropists who plan to go to the Art Commission on Sept. 14 with a new $50 million concept for the site. They have dropped the word museum and rebranded the project “Calder Gardens.” In this version, the exhibition spaces will be almost entirely underground in a series of chambers and sunken gardens designed to make the most of the tiny wedge of land squeezed between 21st and 22nd Streets, directly across from the Rodin Museum.

This new approach emerged from a collaboration between Herzog & de Meuron, the Swiss firm that turned a London power plant into the Tate Modern, and Piet Oudolf, a Dutch landscape architect known for his naturalistic but carefully composed perennial gardens.

Based on renderings made available to The Inquirer, their design is well worth the quarter-century wait.

At a time when Philadelphia architecture rarely rises above the goal of extracting money from land, Calder Gardens promises to be a work of art itself. Ideas about Calder’s creativity and Philadelphia’s history are embedded in every detail. Yet the design is no mere intellectual exercise. The luminous pavilion is intended to function as a portal into a serene subterranean world. It’s a secular chapel where visitors can commune with art and escape their daily cares.

Rethinking the Parkway

Calder Gardens could also help us to think differently about the Parkway.

Ever since Philadelphia cut that grand boulevard through northwest Center City in 1909 — leveling an entire neighborhood in the process — officials have been trying to fill its empty spaces with venerable cultural institutions. That early 20th-century vision, a product of the City Beautiful movement, was already problematic when Rendell first suggested creating a museum celebrating Calder, who was born in Philadelphia in 1898. But because his father, Alexander Stirling Calder, and grandfather, Alexander Milne Calder, were both sculptors whose works adorn the Parkway, the boulevard was seen as an obvious place to locate the building.

» READ MORE: The project seemed dead. But after a 25-year delay, a Calder museum is finally coming to the Parkway.

By the early 2000s, however, Philadelphians were becoming increasingly dissatisfied with the Parkway, which is difficult for pedestrians to cross and lacks such basic amenities as cafés and restrooms. Over the last two decades, planners have repeatedly tried to refashion the boulevard into something more pedestrian-friendly. The opening of the Barnes Foundation in 2012 prompted some improvements, but not nearly enough. The city is now in the midst of another attempt to tame the eight-lane expanse.

Herzog & de Meuron were clearly aware of that history. Their design intentionally subverts the Parkway’s original monumental ambitions by introducing a pavilion that strives to be invisible — or as invisible as a 20-foot-high building can be.

The renderings show a long, flat-roofed structure sinking into a landscape of meadow grasses. The Parkway side would be clad in softly brushed stainless steel panels, with just enough shine to create an Impressionist-like reflection of the plantings. A folded awning, also made of steel, would mark the entrance to the underground galleries. Everything else at street level would remain a manicured green space, laced with walking paths.

The lead designers, Jacques Herzog and Jason Frantzen, repeatedly refer to the main pavilion as a “shed” and a “barn.” Herzog told me their design was inspired by “simple farm buildings found in the American landscape.” At one point, he described American barns as an “innocent form” — a guileless vernacular architecture — and noted that Calder created his large pieces in a barnlike studio at his Connecticut farm. Herzog chose the modest shape, he explained, to avoid turning Calder Gardens into the kind of elaborate architectural eye candy that has become common in so many bespoke designs around the world.

Herzog & de Meuron, of course, are directly responsible for exactly that sort of thrill-seeking architecture. Over the last two decades, the firm has produced the Beijing Olympic stadium known as the “Bird’s Nest,” the Manhattan apartment building nicknamed the “Jenga Tower,” and the precarious, upside-down Caixa Forum in Madrid. But, as Herzog argued when we spoke, the nature of this project, and the tiny 1.8-acre site, demanded a different approach.

How Calder reimagined sculpture

Calder, who is among the most influential artists of the 20th century, produced deeply philosophical works that reimagined what sculpture could be. Yet his art also delights the public in ways few artists can match.

He is so popular that in a typical year, a dozen shows around the world feature his work, attracting more than 2 million visitors, according to Rower at the Calder Foundation. Many of his mobiles and stabiles (the name he gave his earthbound sculptures) are large and need ample space to be displayed properly.

» READ MORE: Where to find 10 extraordinary artworks by the Calder family on display in Philadelphia

Unlike most museums devoted to a single artist, Calder Gardens won’t offer a comprehensive overview of his output. Nor will the project’s organizers, who are led by Philadelphia philanthropists Joseph Neubauer and Marsha Perelman, own any of Calder’s work.

Rather, the Gardens will display a rotating selection of pieces from the Calder Foundation’s extensive collection, lent at no cost. Some objects could remain on display for years, others for just a few weeks. The Barnes will manage the Gardens after it opens, probably in 2025.

The frequent exhibit changes meant Herzog & de Meuron needed to create galleries that were at once specific and general. For such a small venue — just 18,000 square feet — the variety of spaces is remarkable, ranging in mood from modernist to nearly medieval.

After passing through the pavilion lobby, visitors will descend into a partially submerged gallery with 35-foot ceilings. The room, called the Tall Gallery, can accommodate the biggest of Calder’s works. It will also offer views into the two sunken gardens, which will bookend the room.

Because the gallery’s footprint is so small, the architects have carved out several tiny exhibit areas in what would normally be considered throwaway space. One, tucked under a curving staircase, is big enough for just a single object. Another is sheltered under a rocky outcrop in the so-called Vestige Garden. From the look of their rough concrete, these small spaces will likely feel almost cavelike, in contrast to the more formal spaces inside the building.

Herzog and Frantzen went to great lengths to insist that the Calder Gardens design is not an architectural pastiche of the artist’s distinctive aesthetic. While it’s certainly true that the design does not mimic his style, you can still see the architects exploring similar artistic ideas.

Calder had a strong interest in what artists call negative space and loved to twist wire to create outlines of three-dimensional forms. He was essentially sculpting air instead of marble, the material favored by traditional artists like his father and grandfather. The architects’ sunken gardens can also be seen as negative spaces, voids that contrast with the pavilion’s solid volume. It’s probably no accident that they are roughly circular in shape, like the irregular discs that hang from some of Calder’s mobiles.

My favorite is the Vestige Garden. Before starting their design, Herzog & de Meuron made an extensive study of the site, its history and its relationship to the rest of the city. They discovered that rowhouses and factories were simply plowed into the earth during the Parkway’s construction. Calder’s art will sit atop those remains in the same way that the Parkway occupies the former neighborhood. The Parkway was, without a doubt, a great civic undertaking, an attempt to beautify Philadelphia at a time when belching factories and squalid housing defined our streets. But by quietly calling out the lost buildings with the name “Vestige Garden,” Herzog & de Meuron ask us to ponder the Parkway’s lingering effects. Can art redeem its impact?

I-676 incorporated into the design

The architects similarly confront the presence of I-676, which was rammed across Philadelphia’s midsection in the 1980s, further isolating neighborhoods like Chinatown, Logan Square, and Fairmount. Rather than deny the highway’s existence, the Tall Gallery would include a large horizontal window overlooking cruising traffic. Unlike the Parkway, 676 is not meant to be beautiful, but Frantzen argued that “the highway is an important part of the site that should be experienced.” Bounded by the Parkway on one side and the highway on the other, Calder Gardens is the product of these two destructive, but formative, road projects.

The automobile has dictated so much of our Parkway experience. The building that houses the Barnes is an extremely refined work of architecture, yet it is nearly strangled by its parking lot and bus plaza. Happily, Calder Gardens will not be burdened with those accommodations to the car. By displaying the outdoor sculptures in sunken gardens 22 feet below street level, Calder Gardens will also be able to avoid installing a security fence, like the one that surrounds the Rodin Museum across the street.

Calder, nicknamed Sandy, was just 8 when his family left Philadelphia in 1906. With their design for Calder Gardens, Herzog & de Meuron restore his connection to the city with the same spirit of invention, enchantment, and rigor that defined his art. It will be good to have him back.