Near Beijing's $600 million Olympic stadium, migrant worker Ye Yiwen, her husband and two children cram into a tiny underground room, sheltering from the Chinese capital's biting winter and soaring property prices.

Ye's family left behind a 200-square-metre (2,150 square feet) house in a rural outpost 1,000 kilometres (621 miles) away to live in the dimly-lit basement, which -- at 10 square metres -- has just enough space for two beds and one table.

"Of course the house in our old village is more comfortable, but this is where the work is," said Ye, who declined to give her real name.

"And I do miss my flowers," she added with resignation.

The decades-long movement of hundreds of millions of people from China's countryside to its cities is the greatest human migration in history, but those who make the journey do not necessarily find prosperity at their destinations.

About 281,000 people live underground in Beijing according to city authorities, although reports say closer to one million inhabit the capital's basements, former air raid shelters and other subterranean dwellings.

The "Rat Tribe", as they are dubbed locally, are mainly poor migrant workers seeking new opportunities in China's booming cities.

Ye left her village of a few hundred people in the eastern province of Anhui 15 years ago to live in the bustling capital.

A domestic cleaner who works for a family in the Guanjuncheng -- 'City of Champions' -- compound yards from the Olympic village, she brought her sons, now 20 and 21, to Beijing not long after they finished school.

"We don't get in each other's way in our room, although we know it is not ideal," she said.

Migrants such as Ye face formidable barriers in settling in China's big cities under the country's strict household registration system, known as the "hukou".

The ruling Communist party announced plans to let more farmers become urban residents in November, but the soaring cost of property means Ye's family, who make a combined 9,000 yuan ($1,500) a month, will find it impossible to buy.

Prices averaged 31,465 yuan per square metre last month in Beijing, the country's most expensive city, a 28.3 percent leap year-on-year, according to a survey by the independent China Index Academy.

Across 100 major cities new home prices rose 11.51 percent year-on-year to 10,833 yuan per square metre, the figures showed.

Beijing's average home price is 13.3 times average income, state media said last month. The World Bank considers a five to one ratio the limit of affordability, while the United Nations sets it at three to one.

'Saving money is my main concern'

The soaring prices -- a symptom of the widening gulf between rich and poor in the fast developing country -- are a source of discontent with the ruling Communist Party, and not only from migrants.

Guan Sheng, 25, who grew up in the city, sat on his bed in his four-square-metre underground room, searching for jobs on his laptop as the fetid odour from a communal toilet wafted through the door.

Above him a row of drying clothes hung on a steel pipe covered in rust and peeling paint carrying hot water to those living on higher floors -- his only source of heat.

Guan, who did not give his real name, pays 600 yuan ($100) a month for the basement dwelling.

"Saving money is my main concern as far as housing is concerned," he said, walking up a gloomy corridor.

"I typically pay 30 yuan a month for electricity, and there is a set 15 yuan water charge," he said, proud of such bargain accommodation but his tone turning to dejection as he added: "I really can't ever see me owning my own property."

Others in the capital live deeper underground, and in more squalid conditions.

Local media reported last month a group of people living in a sewer. Migrants living there were sent home and the manholes quickly sealed as officials sought to prevent damage to the city's reputation as a booming, prosperous metropolis.

Thousands of air raid shelters were built across the capital after the Communists took power in 1949, fearing attack by Cold War foes.

Their use as accommodation was banned by the government in 2010, but many of the city's poorer residents still make them their homes.

Underneath a building near Guan's lodgings, an elderly woman emerged from a four-square-metre air raid shelter room, taking dirty clothes to a grimy washroom with a long trough urinal, whose condition was reminiscent of that in a decades-old stadium after a packed sports event.

"There are rooms available," she said, rubbing the soaked fabric of a pair of boxer shorts in a plastic basin.

Another voice echoed in the dark: "Why would someone be looking for a room here?"