“This building isn’t meant for art,” Christian Boros said. “How the art fights against the ugly building is very interesting to me.” Photograph by Wolfgang Stahr/laif/Redux

In the heart of Berlin stands a windowless concrete bunker so awesomely ugly that, when you see it, you instinctively avert your gaze. It is heavy, gray, and shrapnel-pocked, and has no signage to explain its protean history. Designed by the Nazi architect Karl Bonatz, under the direction of Albert Speer, the bunker was built in 1942 as an air-raid shelter for German citizens. In 1945, it became a Red Army P.O.W. camp. Later, its cool, sunless chambers served as an East German warehouse for fruit imported from Cuba, which is how it picked up an early nickname: the banana bunker. After the fall of the Berlin Wall, the building was appropriated for use first as an avant-garde performance space, and later as a techno club whose low ceilings, dark rooms, and frequent fetish parties led to its designation as “the hardest club in the world.”

In 2003, a few years after authorities shuttered the night club, Christian and Karen Boros bought the building to display a portion of their sizable collection of contemporary art. They reconfigured its hundred and twenty cramped rooms into eighty larger ones, and added an astonishing window-lined penthouse, where the couple lives with their ten-year-old son. Between 2008 and 2012, more than a hundred and twenty thousand visitors navigated through the bunker’s intricate passageways to see the début exhibition, which focussed on the theme of light. The bunker’s current show, consisting of work from the early nineteen-nineties alongside recent acquisitions, opened in 2012; the couple plans to mount an entirely new show every four years, drawn from their still-growing collection of around seven hundred pieces by eighty artists. “A private collection is not a better model than a museum, but it’s an important add-on,” Christian, who is a publisher and a founder of an ad agency, told me. “You need a museum for historical purposes—to show the best art of the decade, for example. Then you have private collections, with their mistakes, their subjective tastes.”

I visited Christian and Karen in their penthouse, where we sat at a dining table near a large painting by Elizabeth Peyton, surrounded by views of the city. “This building isn’t meant for art,” Christian said. He was wearing gold cufflinks and smoking a Lucky Strike. “How the art fights against the ugly building is very interesting to me.” Karen, who works in V.I.P. relations for Art Basel, guessed that perhaps half of their visitors were more interested in the bunker itself than the art. “We have a lot of artists who people don’t really talk about,” she said. “They may not become part of art history, but they are important to us.”

The bunker is emphatically not a museum. In February, I’d signed up for a ninety-minute group tour on the Web site, which is the only way for a member of the public to gain admittance. I entered the building through a discouragingly heavy, unmarked metal door. During the tour, I marvelled at the diversity of the art on display, from Tomás Saraceno’s delicate floating architectures to an impressive hunk of “We the People,” Danh Vo’s sectile replica of the Statue of Liberty. The modified bunker is perfectly sized to accommodate the large-scale works favored by many contemporary artists—the largest piece, a driftwood sculpture by Ai Weiwei called “Tree,” just barely fits inside the tallest room—and the private tours provide a more intimate experience than what is possible in a museum’s crowded contemporary wing. The quiet setting has also made the collection popular among celebrities. My tour guide confided that when Tom Hanks visited in January, he’d been granted the opportunity to jump into a pile of stale popcorn that is part of an installation. “He is allowed, we are not,” she said.

In part, what makes the Boros Collection so much fun is that it is tailored to the personal and sometimes whimsical aesthetic of its owners. Some of my favorite pieces on display would have been unlikely to survive a museum’s acquisitions committee. The couple has a clear affinity for kinetic sculptures that self-destruct—for example, a slowly disintegrating spinning rubber tire by Michael Sailstorfer. Many of the pieces live outside the realm of art-historical importance or market drama. (“We would never sell a work of art,” Christian told me.) To visit the collection is therefore to invest in a private fantasy of fabulous wealth and confident tastes. At no point can you forget that the patrons who own this building and everything in it are walking around in their penthouse, just above your head.

The Boros Collection is one of three significant, privately owned collections of contemporary art open to the public in Berlin, along with the Haubrok and Hoffman Collections. (There are also private corporate collections, such as the Daimler Collection and the Kunsthalle Deutsche Bank.) Private collections have long existed in the public sphere—the Frick Collection, once a family affair, is a world-famous New York institution—but, in recent years, such collections have increased in size, influence, and renown, particularly in markets, such as China’s, that have a dearth of institutional art spaces and a surfeit of would-be Medicis.

At the same time, the future of the art museum has again become the subject of spirited public debate. In an essay for New York magazine on the renovated Whitney, Jerry Saltz pines for “the museum’s Platonic ideal: a communal effort, conducted over centuries, to preserve, interpret, and commune with artistic ancestors, archetypes, traditions, genres, and methods.” Today’s museum, he writes, is often a “fun-house” of spectacle, more interested in long entrance lines, celebrity, and elegant galas than in the hard work of cultural custodianship. Among some fans of contemporary art, the private collection has emerged as an alternative model to the mega-museum, one that function simultaneously as cultural institution, status symbol, and philanthropic outlet. Yet these spaces have their critics, too. In a recent article for the London Review of Books, Hal Foster writes skeptically of the appeal of contemporary-art acquisitions for neoliberal billionaires. Their collections, he writes, are “auratic as an object yet fungible as an asset. Although they get tax breaks (because they are nominally open to visitors who can book the pilgrimage), these neo-aristocratic institutions don’t pretend to have any real connection to the public sphere. Usually at a remove from urban centres, they are museums of equity display, equal parts prestige and portfolio, and they compete for artwork with institutions that are at least semi-public.”

Foster is probably right to question the motivations of super-rich collectors who claim to act in service of the public good. But the Boroses, at least, had me convinced that their love was pure. They lamented the shallow glamour of the Venice Biennale, whence they’d just returned (Christian: “Is it about art, or is it about watches, luxury bags, and crowds?”), and criticized amateur collectors who do not buy for keeps (Karen: “Some people buy and sell art like shares”). My tour through the bunker was unlike anything I’d experienced in a museum, but, far from bearing out Foster’s critique, the private setting made me all the more enthusiastic and curious about the artists themselves. In part, this was due to the fact that I was able to glimpse the world of private patronage that is so vital to working artists yet rarely addressed in a museum space. “In a private collection, you feel the passion,” Christian said. “We are living with the art.”