The theft of manhole covers is a big problem in China. Tens of thousands are lifted from city streets each year to be sold for scrap metal; in Beijing alone, officials estimated that 240,000 were stolen in 2004.

This can be dangerous — people have died after falling down open manholes, including several toddlers — and authorities have tried various strategies to stop it happening, from covering the metal plates with nets to chaining them to street lights. Yet the problem persists. China is home to a massive scrap metal business that feeds its demand for vital industrial metals — so a nice big piece like a manhole cover can be an easy way to make a little cash.

Now, the eastern city of Hangzhou is trying something new: planting GPS chips inside the covers. City officials are putting 100 so-called "smart manhole covers" on the streets to start. (Credit to Shanghaiist for flagging the story.)

"When a cover is moved and the tilt angle is greater than 15 degrees, the tag will send an alarm signal to us," Tao Xiaomin, an official in Hangzhou's urban management office, told state news agency Xinhua. A "digital positioning system" will allow authorities to track the cover down immediately, Xinhua said.

That authorities would go to the relatively expensive extreme of GPS-tracking manhole covers speaks to both the seriousness of the problem and the difficulty of stopping people from stealing these big metal plates.

This sort of theft isn't unique to China. But the problem tends to be much more common in fast-growing, developing countries — India, for example, is also plagued by manhole theft — which often have huge demand for metals to be used in industries like construction.

China is so hungry for metal that it's at the heart of a multi-billion dollar scrap industry that spans the globe. As Adam Minter, author of the book "Junkyard Planet" explains in his column at Bloomberg, there are two basic methods of procuring a crucial industrial metal like copper: mining it, or processing junk until it's pure enough to be smelted.

China uses both methods, but consumers don't generate nearly enough trash for the country to be self-sufficient in scrap. Metal dealers around the globe sell to China, including US traders who can make millions by collecting and shipping discarded American junk such as old copper wires.

Closer to home, the high demand for scrap has given opportunistic Chinese thieves ample incentive to grab manhole covers. That led Hangzhou officials to another innovation: their new "smart" covers have been deliberately made out of ductile iron, which has a very low scrap value. That might just mean that stealing them isn't worth the hassle.