BIG WAVE GULLY, LANZHOU, China — The hills are alive with the sound of machines. As dozens of excavators gouge the slopes, and countless trucks groan past to dump yellow earth into the valley below, Ye Xingwu pauses from repair work to reflect on their landscape-shattering task in the desolate mountains of the northwest Gansu province.

Despite environmental concerns about the project’s feasibility and long-term impact on this arid, polluted region, a government-hired private developer is slicing the tops off 700 low-level, barren mountains and filling in the valleys to create a 10-square-mile base for “Lanzhou New City,” 8 miles from Gansu’s grimy capital.

“I feel so proud of this project; China is amazing,” says Ye, 40, his hands dripping with engine oil from his excavator. “This is the largest mountain-moving project ever in China. It shows China’s power.”

Critics worry that the rapid pace is steaming past environmental concerns and happening without social reforms crucial to integrating rural migrants. Here in Big Wave Gully, Ye and hundreds of other workers offer a dramatic illustration of how urbanization is transforming China. Hemmed in by hills and the Yellow River, and under government orders not to use arable land, the fast-growing city of Lanzhou has chosen to literally flatten the natural obstacles in the way of progress.

China’s rapid shift from a rural to an urban society is staggering. In 12 years, China will have 221 cities with 1 million inhabitants each compared with 35 cities of that size in Europe today, according to McKinsey Global Institute. By then China is also expected to have 23 cities with more than 5 million people each.

The ratio of Chinese who live in cities has doubled in a little over two decades, from 26% in 1990 to half of the population today.

Yan Jiehe, the multimillionaire founder of China Pacific Construction Group sounds confident about his $3.5 billion investment to carve out a city, ready in five years, “with the flavor of water city Venice, and the flavor of a Las Vegas oasis in the Gobi.” he told China Newsweek, a magazine unrelated to the U.S. Newsweek.

“I am striving to win honor for Chinese people,” says Yan, nicknamed “the Bulldozer.”

Every year, China’s urban population grows by the same size as the population of Australia, about 20 million people, says Tom Miller, author of China’s Urban Billion: The Story Behind the Biggest Migration in Human History.

“Lanzhou is one example of many dozens in China and shows the enormous demand for new housing,” he says.

By 2030, China’s cities will be home to 1 billion people, or one in every eight people on earth, Miller says. China will surpass the United States and cement its position as the world’s largest economy if it urbanizes its once-agrarian population successfully, he says. “But if they get it wrong, China could spend the next 20 years languishing in middle-income torpor, its cities pockmarked by giant slums.”

Muffled in an army overcoat and hat, Liu Hong counts the trucks, as many as 500 a day rumbling in seven days a week, that tip soil down newly created ravines.

“Everyone in my village has a plan to move to the city,” says Liu, 42, a former farmer. “If you try hard, you can achieve it; me, too, if I earn enough money.”

He says the attraction of living in a city is infrastructure, a better environment and more government benefits though China’s household registration system, which divides people into rural and urban residents, would continue to deny his family urban social benefits even after years as city dwellers.

Liu seems not to consider what the nearby city of Lanzhou has become. Ugly, congested and heavily polluted, it is broadly representative of much of Chinese urban life. A January fun run was postponed this year amid runners’ complaints of having to choke their way through the course.

All cars must remain off the roads one day a week to combat smog and gridlock. Taxi driver Zhang Ruijun readily accepts the restriction however ineffective it appears.

“Our city is simply too polluted,” says Zhang, 30, who is confident Lanzhou New City will attract migrants from across the province.

The massive tide of building nationwide is possible because China has a “different economic logic,” than free-market economies, Miller says. Instead of relying on market forces to determine where to expand, “local governments in China have the power to move people, e.g. into a new university district,” he says.

China requires environmental reviews of such projects; it just doesn’t enforce them. Authorities allowed the Lanzhou New City project to begin in October before its mandatory environmental impact assessment report was approved.

Li Ding, a professor of human geography at Lanzhou University, is investigating the project with his students. He fears the development will drain the Yellow River of water that is badly needed for farms and people downstream. Short-term dust storms and long-term landslides are risks as well, says Li, but the developer and officials are reluctant to provide data on the project’s impact.

Most locals dismiss environmental worries.

At the upscale Guofang Baihuo department store, where Gucci ties retail for $255, Luo Xiaolin, 34, who runs a construction firm with her husband, sounds pleased with her city’s speedier growth in recent years after it long lagged its eastern counterparts.

“But air pollution and traffic are getting worse as living conditions have improved so more people have money to buy cars,” she says.

Before climbing back into his excavator, now 75 yards below where he began digging two months ago, operator Yang Shuanglou shares the urban dreams fueled by his $640 monthly wage.

“We want to move to a city. The living conditions will be better there, and I may get a better job,” says Yang, 27, whose wife lives in a north Gansu village. “We will move within 10 years. Maybe I am building our future home here.”