Rat World:The subterranean dwellings of Beijing. Picture Beijing, and a skyline of fancy steel architecture and clouds of smog likely come to mind. But the most fitting metaphor for the city's growing pains may lie beneath its streets: In the past two decades, underground storage basements, parking lots, and air-raid shelters have found new life as apartments, partitioned into untold thousands of cramped, windowless rooms. It's here that many thousands of Beijing's estimated 7 million migrant laborers make their homes, lured underground by low rents ($100 or less a month) and a better -- if unconventional -- life in the capital. Over the past two years, photojournalist Sim Chi Yin has documented this subterranean world known in the Chinese media as home of the "rat tribe," recording the odd mix of quaint domesticity that has managed to flourish within the drafty, moldy cells. Beijingers from every line of work populate these hidden spaces; they are the waiters and hairdressers, fruit-sellers and manicurists of the gilded new capital taking shape above them. Above, 21-year-old Liu Jing sits in her basement apartment in Beijing, China, April 26, 2011. Jing moved to Beijing from the central province of Henan and now works as a pedicurist in east Beijing. Faced with sky-high property prices, living underground is often the only option for this legion of low-waged migrant workers, who make up about one-third of Beijing's 20 million people.