( CNN ) Zhang Qiuli is a pedicurist . Zhao Dan has a room barely wider than a single bed . Zhang Hao has left his one-year-old son thousands of miles away to `` make it '' in China 's capital . Together they are members of what the Chinese press unkindly refer to as the `` rat tribe '' -- people who live underground in a warren of basements and air raid shelters in Beijing . Usually migrant workers , they ca n't afford private housing and , without the official resident permit known as the `` hukou '' they have no access to low-cost government housing , so they find themselves living underground . Estimates suggest there may be more than one million people living underneath the Chinese capital . Photographer Sim Chi Yin has been documenting their lives for the past five years . `` I had a hunch that they were just normal people , '' she says . `` They are actually pretty funky people , most of them are kind of young and all of them have aspirations to move up the social mobility ladder . '' Sim said subterranean living is not as squalid as it might sound . Some use dehumidifiers in summer to take away the damp and in Beijing 's freezing winter months it 's warmer than above ground homes . `` The living space might seem pretty pathetic to us and maybe I went in with this pitying attitude as well , but what I found was the people make the best of their lives down there . '' Annette Kim , a professor at the University of Southern California , has mapped Beijing 's underground city by studying more than 7,000 online rental ads . She found the median size was 9.75 square meters , or 105 square feet , and the mean rent was $ 70 a month -- although she believes the rentals she studied were at the higher end of what 's available . Kim says it 's hard to know exactly how many people live in this type of informal housing . Estimates vary from 200,000 to 2 million . She says 1 million is a reasonable estimate . All buildings constructed in Beijing are required to have basements -- initially a national defense policy that began in the 1950s -- and until 2010 it was perfectly legal to live in these spaces as long as they met building codes . However , the official policy is now to evict people , but it 's not being implemented evenly , Kim says . The number of rental ads for underground housing increased during her year researching the phenomena in 2013 . Alternative low-cost options include living in `` urban villages '' on Beijing 's outskirts . But Kim says : `` They would rather live underground than commute for a long time . It means that sometimes they could have two jobs . '' Kim found the `` rat tribe '' had little interaction with those living above them . `` The people above ground preferred to be as separate as possible and that built up fear of who these people are . '' Zhang Qiuli , the pedicurist , was the first person Sim befriended and photographed . After years spent living beneath a posh condominium in east Beijing , she has since moved `` above ground . '' `` She is one of the success stories , '' says Sim . `` I think for some people there is true upward mobility but for many people the hukou system , whereby migrants ca n't actually buy homes and settle down , is still a huge barrier to them building lives and families here . '' `` Most think they will eventually go home and maybe set up a shop and raise their families . '' CNN 's Anjali Tsui and Kristie Lu Stout contributed to this report .
It 's estimated that one million live underground in Beijing . They 're mainly migrant workers , who ca n't afford private housing . Photographer Sim Chi Yin spent five years documenting their lives .