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Shanghai is one of three cities in the country that have sunk more than two meters since early last century, a situation that could cause problems for underground facilities, such as subway tunnels, according to a recent government survey.

The survey, conducted by the Ministry of Land and Resources, shows Shanghai has sunk by more than two meters since 1921, while Tianjin and Taiyuan, capital of Shanxi Province, have sunk by that much since 1949.

"We have planned to control the annual land subsidence within 5 millimeters by 2020, but it is proving to be very difficult," said Liu Shouqi of the Shanghai Bureau of Housing, Land and Resources Administration.

From September 2002 to September this year, Shanghai sank by around 13 millimeters, 3 millimeters more than the average subsidence in recent years.

Geologists blame the problem on overpumping of underground water and the rapid construction of skyscrapers in the city.

Shanghai's subsidence problems was at it worst in the 1960s, when the city sunk by more than 10 centimeters a year, a rate that would have put the city below sea level by 1999 if it hadn't been slowed.

Just last week, the city passed an urban planning law that limits the height and density of new buildings constructed downtown.

The city has also taken to pumping water back into underground reservoirs to make up for the overuse of underground water sources.

But attempts to limit the amount of water pumped out of these reservoirs have proven unsuccessful. The city set an annual pumping limit of around 50 million cubic meters, but more than twice that much is draw from underground sources each year by factories, which use the water to cool equipment, and farmers.

"It's not easy to stop the overpumping because the users of underground water exist everywhere from downtown to rural areas, and there isn't one clear government department in charge of the problem," Liu said.

"Although the current (subsidence) rate will keep the city above sea level for more than 400 years, it still holds several potential dangers," said Li Qinfen, a professor at the Shanghai Institute of Geological Survey.

"It has already forced the city to raise its floodwall again and again," he said. "And it will also very probably cause the city's subway lines to deform because of the unevenness of land subsidence."

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