In Beijing, anywhere between 100,000 to one million people live underground in old bomb shelters. Dubbed the shuzu, or "rat tribe," these subterranean citizens occupy a cramped, musty, and windowless world located dozens of feet below the bustling streets of China's capital city. This extensive network of largely illicit bunker housing is unlike anything found in any other major metropolis in the world.

The roots of Beijing's invisible underworld were laid during what could be called the other Cold War. In 1969, tensions between China and the Soviet Union escalated, with the two communist-led countries clashing at the Sino-Soviet border. When Chinese troops ambushed Soviet border guards that year at Zhenbao Island—a disputed territory located in the middle of the Ussuri River separating northeastern China from Russia's Far East—the hostilities turned bloody, prompting both countries to prepare for a possible nuclear attack. In China, Chairman Mao Zedong advised his cities to build nuclear bomb shelters. Beijing responded by constructing approximately 10,000 bunkers.

By the late 1980s, China's government had started to liberalize and tensions with the Soviet Union had cooled, leading the Office of Civil Defense to lease these shelters to local landlords, who in turn began leasing the spaces to desperate migrant workers and young people. For many, living dozens of feet underground in a windowless bunker was the only way to chase their dreams of scaling the social ladder. That remains true today.

It’s a familiar tale: The cost of living in Beijing is high and still rising. With more than 21 million people now calling the city home, it is among the world’s most expensive places to live. The rising cost of rent far outpaces the average person's income, yet people continue to flock to the area because it brims with social and economic opportunity. “[W]ith limited access to public, affordable housing, nuclear bunkers are one of the few feasible options for migrant workers,” Ye Ming writes for National Geographic. A small, shared dorm in a concrete bunker can cost as little as $20 per month.

For many, the central location makes these bunkers worthwhile despite the lack of space and sunlight. As Annette M. Kim, an associate professor of public policy at USC, writes in the academic journal Cities, “[T]he priority for the lower-income, often migrant population in Beijing is for rental housing located in the central city. The ability to walk and/or bike to jobs as well as low rents, both of which allow for the possibility of accumulating savings, is worth making the choice to live in small underground rooms.”

As you might expect from a converted nuclear fallout shelter, the spaces have some of the basics—plumbing, sewage, and electricity—and very little of anything else. There’s no natural light, there's very little ventilation, and most amenities, such as kitchens and bathrooms, must be shared with neighbors. And while Ming reports that local law requires apartments to have at least 43 square feet per tenant, that rule is clearly not enforced. Some apartments might as well be closets.

But as Kim explains, super-dense housing conditions aren't unique to Beijing. “[T]his is not an idiosyncratic situation. History shows that immigrants coped by living in crowded basement units as well as tenements during the west’s rapid urbanization.”

The question is whether that trend should—or will—continue in the future. In 2010, the city announced a ban on residential use of nuclear shelters, but the decree has done little to stop people from making their homes there. “If it is desirable to not allow people to live underground, we are challenged with the task of finding other spaces for roughly a million people,” Kim writes.

For foreigners, it can be difficult to gain access to this underground shelter city. In 2015, the Italian photographer Antonio Faccilongo managed to sneak below, capturing life in the bunkers for a series entitled Atomic Rooms. For a look inside, you can view his work here.