A gleaming metallic wall cuts through a scrubby hillside along a Philadelphia highway like a giant steel blade slicing into the earth. Halfway down its length, this silvery barrier flips upward, resembling the lid of an enormous laptop and creating an entrance canopy that invites visitors inside. This otherworldly structure marks the entrance to Calder Gardens, a stunning new cultural complex designed by renowned Swiss architects Herzog & de Meuron to celebrate the work of Philadelphia-born Alexander Calder, the master of mobile sculptures.

As visitors climb the planted hill, they discover deep furrows carved into the ground and jagged concrete sinkholes from which colorful sculpture peaks emerge. The $90 million project represents one of the most unusual cultural complexes built anywhere in recent years. Despite occupying a site no larger than a football field, wedged between two busy highways, the facility offers visitors a captivating journey of discovery that takes them deep underground through spaces that are part barn, part cave, and part rolling meadow.

"I have never worked on anything like this before," says 75-year-old Jacques Herzog, who has created numerous museums and galleries worldwide, often reinventing the architectural type each time. "There was literally no brief. I felt like an artist, waking up every morning without someone telling me what to do. Architecture is never this free." The lack of a defined brief reflected the client's unconventional vision for the space.

Sandy Rower, Calder's grandson and president of the Calder Foundation who led the project, had strong opinions about what he didn't want. "I had no intention of making a museum," Rower explains. "We wanted people to be able to sit in resonance with the work and have their own mysterious, unmediated experience. My grandfather wasn't trying to predetermine the viewer's reaction, so we don't want to tell people what to think or feel."

The project addresses a long-standing absence in Philadelphia's cultural landscape. Born in 1898, Calder came from the third generation of a distinguished Philadelphia artistic family. His grandfather sculpted the statue of William Penn that crowns City Hall, while his father created a striking fountain of reclining river gods that stands on a nearby roundabout. However, the younger and most famous Calder left the city at age eight and, aside from a large mobile in the main art museum, had never maintained much of a presence in his birthplace until now.

Rower describes the project as something of a spiritual quest, referring to the complex as a "hypogeum," meaning an underground temple or tomb, and calling it "a sacred space for self-cultivation." The facility indeed carries a ritualistic atmosphere, taking visitors on a theatrical journey of compression and release. Guests are guided through dark passages, then suddenly thrust into unexpectedly airy galleries, invited to peer around corners, perch in small alcoves, and explore sunken gardens to discover the artwork on their own terms, with not a single wall text in sight.

The arrival experience contrasts dramatically with the nearby neoclassical cultural buildings that line Philadelphia's Parkway Museum District, which is actually a misnomer for an inhospitable ribbon of highways that sliced through this part of the city in the 1960s. After crossing 10 lanes of traffic, visitors arrive at a hilly garden (unfortunately surrounded by an unattractive chain-link fence) and climb one of several winding paths through what will eventually grow into a lush display of perennials, designed by Dutch celebrity plantsman Piet Oudolf, finally reaching a circular plaza at the summit.

Upon entering beneath the building's sharp steel flap, visitors find themselves in a warm timber-lined lobby with the feel of a domestically-sized Apple Store. A bleacher staircase leads down to the first glimpse of a sunken gallery, where the twisted, spider-like limbs of one of Calder's "stabiles" (his term for static sculptures) stretch out in taut arcs. A mobile dangles above, hanging like a splatter of black paint frozen in mid-air. The rumbling interstate highway can be briefly glimpsed through a long horizontal window, providing a reminder of the outside world that visitors will soon forget as they descend deeper into the earth.

The experience becomes increasingly surreal at a second staircase, which winds down through what feels like a lava tube, lined with bubbling black sprayed concrete resembling rugged basalt. A circular opening carved through the wall provides a window onto another Calder mobile, hanging spotlit in the dark gloom like a ghostly white tangle of coat hangers. These dark stairs lead to a light-flooded central gallery topped by a polished plaster ceiling that droops in a shallow bulge overhead, like the underside of a puddle, adding an unsettling atmosphere as if the weight of the ground above might burst through at any moment.

In the central space, one of Calder's red steel sculptures writhes in the middle with twisted gymnastic curves, while more mobiles hang nearby like flocks of angular birds. Windows on either side provide views of additional artworks. In one direction, a balancing black-and-red object stands imprisoned in a cylindrical concrete well, ringed by a dimly lit curved corridor gallery where paintings and drawings hang on raw foundation walls. A side niche presents another mobile against a seamless white backdrop with the depthless quality of a photographic studio, while another purple-painted niche houses a sort of family altar with works by Calder's parents and grandfather.

On the other side of the gallery, a door leads to a sunken garden where the site's historic street pattern is recalled through angular concrete forms jutting into the courtyard, their surfaces scraped with a rough, rocky finish as if freshly clawed from the earth. Flood risk prevented the use of rammed earth, so concrete is used throughout to simulate stone and soil. "Calder's work was about negative space," Herzog explains, "so our process was one of excavating and carving, rather than creating positive form. We see the project as an organism with different members, each with its distinct character, triggering unlikely situations."

The complex displays a palpable restlessness throughout, sometimes feeling like too many ideas crammed into too small a space—a virtuosic display of Herzog & de Meuron's greatest hits populated by incidental Calders. The design seems very much tuned to our impatient age of spectacle, and the theatrical, staged quality can sometimes verge on Disney-like territory. The absence of any explanatory text can prove frustrating, though Rower insists he doesn't want visitors staring at their phones.

Juana Berrío, director of programming, explains that they're developing alternative forms of interpretation. "Sometimes it will be music, or a sound bath beneath one of the sculptures, or collective singing," she says. "It's about celebration, experimentation and real-time experience." The loaned works will be in constant rotation, ensuring that each visit offers something new to discover.

While visitors may leave feeling entertained but not necessarily more knowledgeable about Calder, perhaps that aligns with the artist's own philosophy. "Theories may be all very well for the artist himself," Calder once said, "but they shouldn't be broadcast to other people." There's a childlike wonder to both Calder's work and the building Herzog & de Meuron created to display it—even extending to the restrooms, which are bathed in rich blue resinous material. Children will undoubtedly love exploring this labyrinthine space, just as they've stared in wonder at Calder's mobiles for generations. As the artist once quipped, "My fan mail is enormous: everybody is under six."