China's campaign to bulldoze mountains to create land to build on could cause extensive environmental problems, scientists say.
Researchers from Chang'an University in China have warned that dozens of mountains have already been flattened - and this is causing air and water pollution, soil erosion and flooding.
They say that this activity is happening on an unprecedented scale.
They report their concerns in the journal Nature.
Prof Peiyue Li, from Chang'an University's School of Environmental Science and Engineering, said: "Because there have been no land creation projects like this before in the world, there are no guidelines."
China's cities are expanding rapidly as its economy grows, and moving mountains is one way to supply more land for development.
About one-fifth of the country's population lives in mountainous areas.
Around the country, in cities such as Chongqing, Shiyan, Yichang, Lanzhou and Yan'an, dozens of hilltops have been levelled.
The soil and rock is then used to fill in valleys, and overall this has so far created hundreds of square kilometres of flat terrain.
Prof Li said: "Mountainous cities such as Yan'an are mostly located in relatively flat valleys.
"The valleys are narrow and limit the development of the cities - and huge population density is also a factor."
While mountaintop removal is sometimes used by the mining industry, particularly in the US, researchers say the scale of this in China is unparalleled.
They warn that turning hills into plains is throwing dust particles into the atmosphere, polluting waterways, causing landslides and flooding and endangering plants and animals.
They add that the flattened land could also be unsuitable to build on.
Prof Li explained: "The most concerning issue is the safety of constructing cities on the newly created land.
"Yan'an, for example, is the largest project ever attempted on land that is composed of thick windblown silt.
"Such soft soils can subside when wet, causing structural collapse and land subsidence. Building on such soils is quite dangerous and it would take a very long time for the ground base to become stable."
The scientists say that the Chinese government should work with national and international experts to fully assess the risks before they continue.
Commenting on the issue, Prof Brian McGlynn, from Duke University in the US, told BBC Radio 4's Inside Science programme: "In the US and China, we're moving ahead without much insight into what the result will be, especially when it comes to the water, the hydrology, the water quality implications.
"The [comment] article focuses on the structural issues, the ability of the land to stabilise. In addition to that we're massively changing the flow of water and material it comes into contact with.
"We don't have any experience with manipulations on this scale: It's a large experiment."
Dr Jan Zalasiewicz, from the University of Leicester, added: "We're in new territory with these kinds of changes.
"There are other projects as well, like the Palm Island in Dubai, which is moving billions of tonnes of materials in one place to another to create a new landscape.
"And while humans have been doing that on a small to moderate scale for quite a long time, this is now exceeding the state of natural processes."
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