In Malibu, an Inflatable Bungalow for Robert Downey Jr.

A BREEZE CUTS through the ocean air as a mechanical gate opens onto Robert Downey Jr.’s seven-acre Malibu, Calif., estate. Down the gravel driveway, a tennis court overlooks Zuma Beach; to the right, alpacas rest on a sunny patch of grass not far from the cottage-style main house where the actor lives with his wife, Susan, a 48-year-old movie producer, and their two young children. But the most expressive signifier of the Downeys’ whimsy sits in the other direction, behind the kidney-shaped swimming pool: an undulating 6,500-square-foot white concrete domed structure that could either be prehistoric or from the distant future. Downey, 57, calls it the Clubhouse.

“It was a crucible of faith,” says the actor about his elaborate version of the typically humble Binishell, a form originated in 1964 by Dante Bini. The now-90-year-old Italian industrial designer discovered that by topping a nylon-coated neoprene air bladder with wet steel-reinforced concrete, then slowly inflating it, he could make — in an hour or so, about the time it takes for the material to cure — a naturally aerodynamic and durable thin-shell bungalow. Downey’s Binishell was designed by the creator’s son, Nicolò, a 55-year-old Beverly Hills-based architect who’s reimagining his father’s innovation. Nicolò didn’t so much follow in the elder Bini’s footsteps as stumble into them while puzzling over fast, efficient solutions to the global housing crisis. “My intention wasn’t to propagate his work,” he says. “It was to figure out a better way of building.”

At the height of their popularity in the late 1970s, Binishells popped up like soap bubbles across Australia, where Bini, who got the idea for the domes after a game of tennis under an inflatable roof, had been hired by the public works department of New South Wales to build schools; his prototypes were adapted for homes, gymnasiums, libraries and shopping centers. The structures were also erected as grain silos in the deserts of Afghanistan and to protect seismographs atop Mount Etna in Sicily, where, through earthquakes and eruptions, one remains standing 50 years later. Bini’s favorite may be La Cupola in Sardinia, a now-derelict cliff-top monument to the love affair between Michelangelo Antonioni, a master of Modernist Italian cinema, and his muse, the actress Monica Vitti. “They wanted to live in a sculpture,” says Bini, who once accompanied the director to a local quarry to smell “the scent of the universe” in a freshly split slab of pink granite.

The Downeys decided to have Nicolò build them their dome in 2013, just as the “Iron Man” trilogy was coming to an end. They wanted a stand-alone place for entertaining and to house guests, and were underwhelmed by other architects’ restrained proposals. A mutual friend, the English designer Robert Clydesdale, suggested they consider Nicolò, who had been working to update his father’s Binishells — and bring them up to code — on a remote plot of land in Joshua Tree. The actor was skeptical at first but eventually embraced the idea. “We wanted to try something that hadn’t been done before,” says Susan. “Plus, we love a character.”

“KINDLY BRING ME one raw egg,” says Nicolò to a waiter at a splashy members’ club in Hollywood. It’s a clear March night, and the skyline has come alive with a million little lights. When the egg arrives on a platter, he squeezes the ends of it between his fingers and thumb to illustrate the architectural soundness of the Binishell, which, over the course of dinner, he compares to Palais Bulles on the French Riviera, Istanbul’s Hagia Sophia, the Pantheon in Rome, an athlete crouched low to the ground, an exhaling pair of lungs, a baby’s bottom, the miracle of childbirth and a hug. “Call it biomimicry or call it borrowing from nature, but the beauty is that it works,” he says.

Erecting it turned out to be the easy part. In June 2015, on the morning it was inflated, Downey walked over to lay crystals at the foundation and, as the couple stood together to watch the inner liner get pulled away, for just a second, magic overtook reason. But there was still the matter of how to finish the interior and furnish it. Nearly two years after its dramatic birth, the shell sat dormant as the Downeys struggled with what to do next; construction teams bailed, insisting the structure would eventually collapse. Some nights, the actor would run a cable line into it from the main house so that he and his friends could watch U.F.C. fights on folding chairs. Susan’s brother begged Downey to give up on the clubhouse and turn it into a garage.

In the interim, the couple found themselves increasingly committed to ecologically focused solutions to social problems. (In 2019, Downey created the FootPrint Coalition, a venture fund that invests in sustainable-minded companies. They’ve used their land as a testing ground for technologies like solar-generated water systems and a pair of wind turbines to offset the building’s energy consumption.) Eventually, after many false starts, they convinced Joe Nahem of the New York-based firm Fox-Nahem, who had worked on their house in the Hamptons, a 19th-century windmill, to take on the interior architecture and design, which were, he concedes, “very intimidating.”

And yet the challenge of creating such an architectural oddity — a Buckminster Fuller folly by way of Beverly Hills — was clearly part of the fun. Through the recessed main entrance with an oval-shaped, pivot-hinged blue door, the Downeys’ Binishell opens dramatically into an expansive foyer with a saltwater aquarium. A wavy screen made of fiberglass beads in gray, brown and neon yellow separates a breakfast nook from the dining room; nearby, a jokey display case contains a pair of fake-marshmallow-tipped sticks with instructions to “break glass in case of fire.”

For a home lacking interior walls, at least structural ones, the dome feels surprisingly cozy; where the plaster-finished gypsum room dividers don’t extend all the way to the curved ceiling, panes of soundproof glass have been added for privacy. From the kitchen, the heart of the structure, a hallway leads past a space-themed arcade game into a bright living room with cork flooring. Another hall, lined with a fully stocked candy bar, leads to a self-contained screening room accessible through a folding garage-type door. There are two guest bedrooms — both with a view of “Toxic Mickey” (2017), a bronze fountain sculpture by the American artist Bill Barminski that depicts a man chest-deep in a punctured oil drum, wearing a gas mask in the shape of Walt Disney’s beloved mouse.

Seeing their Binishell — a refuge carved from an alien-seeming pod — today, it’s hard to imagine this wasn’t the couple’s plan all along. But without the Downeys’ dedication to the quixotic project through almost a decade, there was a chance it might have wound up a noble but failed experiment, little more than a place to stash spare sports cars and lawn maintenance equipment. “Let me put it this way,” the actor says. “If this were a movie with a budget that we’d been charged with producing, we would have been fired 12 times.”

Photo assistants: Nik Williams, Jeff Desom