By Martin Patience
BBC News, Beijing
Jong Li Na shares an apartment only just bigger than her bed
Millions of people have poured into the Chinese capital Beijing from the countryside, but with not enough homes to go around, some end up living underground in old air-raid shelters, basements and tunnels.
Even by Beijing's standards, a sprawling city of almost 20 million, it was not the easiest of addresses to find.
We arrived at a 20-storey apartment building on the outskirts of town, where the subway lines stopped and huge cauldron-like power stations reared into view.
But instead of taking the lift up into the apartments, we entered an easy-to-miss doorway, and descended a dark flight of stairs. The echoes of our footsteps flooded our ears.
Then, in front of us, was a thick steel door with two bolts across it. It was the only clue that this was an air raid shelter. It was also a place that dozens of people called home.
Walking along the dark and dingy corridors, you could hear the muffled sounds of life.
We had arranged to meet two young women living in the shelter. One was Jong Li Na, a tough 18-year-old who was trying to make it in the big city.
A rapid house-building project is under way in Beijing
She had found work in a restaurant as a waitress, but had quit her job and now was trying to find something better.
An old colleague, Peng Jing, was her room-mate. She was from southern China, and had a loud, throaty voice which boomed off the walls.
Their home was one tiny room. There was a single bed, which they both slept in, and enough space for a small chest of drawers.
They shared three toilets with the 100 or so other people living in the shelter.
For these damp surroundings, the women paid about £30 a month.
If you wanted to hire a TV that would be £5 extra. To get reception for that TV, that would be another £1.50 for the month.
Perhaps unsurprisingly, Li Na said the shelter was affecting her health.
At least twice a day she would leave her room and go outside. She would suck in the air and watch the city. Normal life above ground.
No-one knows for sure how many people live in the shelters and tunnels, bunkers and basements of Beijing. They are sometimes called the "mouse tribe", and could number one million.
Most are migrants, sweeping into China's cities in the last decade or so, the largest wave of urbanisation in human history.
In the past affordable homes have gone to officials or property developers
Priced out of the property market, they have been living under the pavements in a city of rapid change.
Beijing has seen its population swell by more than six million in the past decade.
Little wonder the authorities have run into some difficulties. There was a ruling last year that those living in the city's air-raid shelters were to be moved out. But like many rulings here, there has been little follow-through.
Chinese authorities aim to build 10 million new affordable homes across the country this year. In the next five years, they hope to build a total of 35 million. The numbers, as always in China, are astonishing.
These new apartments are to be aimed at poor and middle-class families to either rent or buy.
Well, that is the idea. But the track record is not particularly good.
In the past, many of these affordable homes have been snapped up either by government officials or property developers, who have then sold them on at enormous profit.
Emerging from below
Back underground, Li Na and her friend prepared for a night out.
Li Na avidly did her make-up - putting on blusher, holding up the mirror, and then put it down on the folds of the bed sheets. Every few minutes, she would re-do it.
They were not planning to do anything fancy, as they did not have much money.
Adverts for underground rooms available for rent are posted on walls
They would probably eat street snacks and then wander to a local park, where the elderly often gathered to dance in the evenings.
Being underground, of course, you have no idea what the weather is like upstairs.
I did not have the heart to tell them it was raining (extremely rare for Beijing) and so there would not be any dancing.
But even then, I doubt it would have dampened their enthusiasm.
Despite starting at the very bottom, Li Na was determined to make the best of her life.
"When I look up at the high buildings, I see the neon lights," she told me.
"I see the hundreds of windows and homes and of course I dream that one day, I'll buy my own apartment."
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