Public space must be as free and abundant as the air we breathe. In our real estate-driven cities, where open space is increasingly carved up, traded and sold as a commodity, architects must defend public space, advocate for more of it, and reclaim space that's been squandered by neglect or lack of vision. In our practice, this sometimes means openly sparring with a client to carve out public space, or inventing stealthy, under-the-radar ways of insinuating space for the public into otherwise private building projects. Either way, all democracies need champions. It's our role, as stewards of the urban realm, to will public space into existence and to democratize our progressively privatized cities. In 2004, my studio came into the orbit of two inspired citizen activists who launched a campaign to save a 1.5-mile stretch of derelict infrastructure and convert it into a public park. After years of struggle and mounting pressure from local developers, the High Line was saved from demolition, and we, along with our partners James Corner and Piet Oudolf, were put in charge of designing it. We fell in love with the accidental ecosystem that developed there after years of neglect. Rather than making architecture, we vowed to protect this place from architecture. The site was too fragile to share with the public, so we reinterpreted the DNA of this weird, self-seeded ecosystem that was half natural and half man-made, into a hybrid we called agritecture. Typically, parks serve as an escape from the city. But this park was conceived as an entry into the city, a portal into the city's subconscious. Floating over the fast-paced streets below, the High Line became a place to experience an alternative New York, with views that could never make it onto a postcard. In a culture that rewards relentless productivity, the High Line became a parenthesis in the day for doing nothing but sharing in the pleasures of being urban. Unexpectedly, the High Line became one of the most popular destinations in New York and a landmark on the world tourist map. Last year, over eight million people came. The High Line also went viral. Hundreds of cities around the world were inspired to build one of their own. We touched a global nerve. In a time of environmental awareness and shrinking resources on the planet, cities realized they could seize the opportunity to reimagine aging infrastructure as a sustainable way to give back space to the public. After all, access to green space is an environmental justice issue. In 2013, we were selected to design a park in central Moscow. Thankfully, the city pivoted from its plan to build a giant commercial development on this historically sensitive and politically charged site, adjacent to the Kremlin, Red Square and St. Basil's Cathedral. It would sit on the footprint of the former massive Khrushchev-era hotel "Rossiya." We faced a moral dilemma. Was it possible to make a democratic public space in the context of a repressive regime? Despite being a stone's throw from the Kremlin, we decided to focus on Moscow's aspirations of becoming a progressive, cosmopolitan city. As national governments are failing us, cities hold the promise of social reform. The park would be a site of civic expression, a foil to the military parades and other demonstrations of power in Red Square. Given the vulnerability that public spaces pose from opposition, governments try to control them. The architectural brief we got discouraged large open spaces, presumably out of concern for public assemblies and social unrest. Our response was to make open meadows and plazas whose uses could be open-ended. Instead of the manicured gardens and restricted inventories of official plantings, like rose bushes, we introduced a principle we called wild urbanism. The park would host native plants, sourced from the four major regional landscapes of Russia. This was our stealthy move. It was embraced as an expression of national pride. In contrast to typical parks in Moscow, where you're only permitted to walk on pathways, fenced off from vegetation, this park is unscripted and encouraged immersion in the landscape. Zaryadye Park has been immensely successful. One million people came the first month. So, not surprisingly, Putin politicized Zaryadye as his park for the people. Meanwhile, the park's liberating effect on a repressed younger generation was caught on security cameras. Government officials blamed American influence for corrupting Russian youth. But for us, this was a great sign of success. We came to believe that regimes come and go -- some more slowly than others -- but public spaces endure. They can work quietly, even subversively, to empower the public. The threat to democratic public space comes also from financial greed. Returning to New York, the neighborhood surrounding the High Line had transformed from a sea of open parking lots to the most expensive real estate in New York. The park inadvertently fell victim to its own success and became an agent of rapid urbanization. And with it came gentrification. I question what is the responsibility of the architect in shaping the aftermath of urban change that they've unwittingly produced. I felt compelled to respond on the site where it happened, to use the public space of the High Line as an urban stage for an epic performance called "The Mile-Long Opera." It would be a meditation on the unprecedented speed of change of the postindustrial city, its winners and losers. And it would embody a sense of nostalgia we feel for an irretrievable past and apprehension about an alienating future. People tend to think of opera as expensive and exclusive. This would welcome everyone for free. I stepped into the role of creator, director and producer, and basically off a cliff, but I brought some brilliant collaborators with me. "The Mile-Long Opera" was performed by a giant ensemble of 40 church, community and school choirs. One thousand singers in all were distributed along the 1.5-mile stretch of the High Line. (Singers singing opera) Elizabeth Diller: Each singer performed solo to a promenading audience of thousands each night for seven nights, each expressing their unique way of coping with contemporary life. Through anxiety, humor, longing, vulnerability, joy and outrage. The city was their backdrop. During some particularly dark days of political strife in the country, across a big swath of Manhattan, there was a palpable sense of shared values and citizenship among New Yorkers. But development around the High Line was not slowing down. A huge real-estate play called Hudson Yards was in the process of becoming the largest mixed-use development in US history. In its wisdom, the city of New York retained a small piece of that huge property for a yet-to-be-determined cultural facility, and asked for ideas. And while not the ideal spot, we thought, "Why not be opportunistic? Why not use the space produced by commercial development for countercultural activity?" With our partner David Rockwell, we had a vision for a building and an institutional ethos. The new entity had to be responsive to an unpredictable future in which artists would be free to work across all disciplines and all media, at all scales, indoors and out. To do so, we had to change the paradigm and challenge the inertia of architecture. Made up of a fixed building with a stack of multi-use galleries and a telescoping outer shell that deploys on demand, The Shed is able to double its footprint for large installations, performances and events. If you don't need the extra space, you can just nest the shell and open up a large outdoor space for cultural and public use. The structure deploys in five minutes, and uses the horsepower of one car engine. The Shed is a start-up realized with a group of visionary collaborators based on a hunch and sheer will. (Music) While it's a small pocket of resistance on publicly owned land and a giant commercial site, The Shed asserts its independence strongly through its content. As populations expand and city growth is inevitable, it's important for those of us who build to relentlessly advocate for a democratic public realm so that dwindling urban space is not forfeited to the highest bidder. Thank you.