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."

(CNN)