(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 can'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."

Underground city

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.

The orange dots show underground housing in Beijing. The blue dots are affordable housing.

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."

Moving up

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 can'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."