People have known about China's "ghost cities" for years, but it turns out finding and tracking these massive, mostly empty housing developments is pretty much as difficult as hunting an actual ghost.

That is, until China's largest search engine got involved, Quartz reports. Tech company Baidu was able to identify 50 ghost cities by spending months studying data from 770 million users.

Any residential areas with less than a quarter the number of expected Baidu users were classified as a ghost city. It's thought to be the most accurate image of these empty towns ever created.

"Now that we know where the ghost areas are, we can try to identify why these areas are 'ghostly' and do something about it," Baidu researcher Haishan Wu tells NPR.

China's ghost cities sprang up in the past decade through a combination of a housing shortage, a real estate boom, and laws that allow cities to buy cheap rural land and sell it to developers for huge profits, NPR reports.

But some developers misjudged demand in certain areas, leaving housing empty and feeling like—in the words of one NPR staffer who visited a ghost city—"an abandoned movie set." Part of the problem is that a lot of the housing consists of just concrete walls, often lacking even a toilet.

And the surrounding neighborhoods don't boast much in the way of amenities, either. Wu tells NPR people probably don't want to live in places without schools, hospitals, or malls.

However, urban planning experts believe China's ghost cities can be returned to the land of the living with a little time and effort. (China, meanwhile, found a vast gold deposit—but there's a catch.)

This article originally appeared on Newser: China's 'Ghost Cities' Were Hidden Until Now

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