Deep below Beijing is an underground world of bomb shelters where one million citizens currently reside.

Beijing's collection of 10,000 concrete bunkers was originally a project of Mao Zedong in the 1960s, an integral part of his Cold War defense strategy, according to a report from New York Times. It's reportedly strong enough to withstand nuclear bombs. However, this proved unnecessary when the 1980s saw the shelters turned over to the authorities, who converted them to shops, offices and living spaces.

Nuclear Bunkers a Solution for High Rents  

Migrant workers, students and other residents who opt to live belowground often do so to escape the rising costs of the Beijing market. After all, the units in the nuclear bunkers cost roughly a third of what a residence would cost aboveground, around 500 to 900 renminbi ($77 to $138) a month.

Unfortunately, the living conditions in the bunkers reflect the low prices -- and the number of people squeezed down there, estimated at around a million, don't help.

Harsh Underground Living Conditions 

Italian photographer Antonio Faccilongo visited the underground world in December 2015, sharing with National Geographic his photographs and insights of the unusual living conditions of over a million people who call the atomic shelters home. As a foreigner, he was denied access and slipped in unseen during the security guards' lunch break.

Faccilongo discovered the harsh conditions of life underground despite being equipped with electricity, plumbing and a sewage system. There's no proper ventilation, and shared spaces like kitchens and bathrooms tend to be unsanitary and crowded. While local laws require a minimum of four square meters per tenant, this is not strictly implemented in the bunkers.

Most of the residents were hesitant to be photographed.

"I met around 150 people, and only 50 gave me permissions [to photograph them]," Faccilongo said. "Some of them are afraid because they told their families [back home] that they have good jobs and are living in good apartments."

Officials have attempted to prohibit using these nuclear shelters as living spaces, but there is the problem of where to put one million displaced citizens.